design/ + source rustdoc: replace opaque shorthand with content phrases + links

Reader-facing prose and rustdoc carried opaque shorthand like
"Decision 10", "clause-5", "mq.1", "ct.1", "eob.1", "rpe.1",
"post-mq.3", and "Iter 22b.1:" with no in-repo definition the reader
could follow. This commit replaces every such occurrence in the
durable tier the reader is most likely to land on (design/ ledger +
source //! module headers + the central /// public-item rustdoc) with
an inline content phrase plus, where applicable, a Markdown link to
the file that defines the referenced concept.

design/ ledger — 16 files:
  Definition-site headings demoted from "Decision N: <title>" to
  "<title>": authoring-surface, tail-calls, memory-model section in
  rc-uniqueness.md, dual-allocator section, typeclass design,
  effects "pure core + algebraic effects".
  Cross-reference sites: "Decision 1" -> canonical-schema principle
  (data-model); "Decision 3/4" -> effects + scope-boundaries; "Decision
  6" -> authoring-surface; "Decision 8" -> tail-calls; "Decision 9" ->
  rc-uniqueness (dual-allocator); "Decision 10" -> memory-model;
  "Decision 11" -> typeclasses (model). "clause-5" -> body-link
  durability gate. "clause-3" (in language-constraints) ->
  bug-class-reintroduction discriminator. "mq.1/2/3", "ct.1/4",
  "eob.1", "rpe.1" -> the canonical-form rule / the type-driven
  dispatch / the Str carve-out / etc. "post-mq.3" -> "type-driven".

design/contracts/feature-acceptance.md: file-local "clauses 1/2/3"
-> "criteria 1/2/3" (sprachliche Kohärenz mit der File-Überschrift
"Feature-acceptance criterion"); "the clause-3 mechanism" -> "the
bug-class-reintroduction discriminator".

Source //! module headers — 24 files:
  Stripped "Iter X.Y:" prefixes and "(Decision N)" / "(mq.X)" tags
  from spec_drift, uniqueness, reuse_shape, migrate_canonical_types,
  typeclass_22b{2,3,c}, suppress_filter, lift, mono, linearity,
  diagnostic, method_dispatch_pin, method_collision_pin,
  no_per_type_print_ops, mq3_multi_class_e2e, print_mono_body_shape,
  print_no_leak_pin, cli_diag_human_workspace_load_error,
  ct1_check_cli, prose snapshot, unbound_in_instance_method_pin,
  mono_xmod_ctor_pattern, desugar.

Central /// public-item rustdoc:
  ast.rs (full sweep — every "Iter X" + "Decision N" prefix
  reformulated; mode/Type::Fn rustdoc now points at memory-model.md;
  Constraint / SuperclassRef / InstanceDef / ClassDef rustdoc points
  at typeclasses contract).
  diagnostic.rs (all "(Iter X)" / "(mq.X)" tags on diagnostic codes
  removed).
  lib.rs (FORM_A_SPEC rustdoc points at authoring-surface.md
  instead of "Decision 6").
  canonical.rs (type_hash + Float-literal rustdoc).

Still outstanding (for a follow-up commit): ~500 inline `//`
code-body comments with `Iter X.Y` markers across the workspace, and
a handful of `///` rustdoc items in hash_pin / workspace_pin / lift /
mono / suppress_filter test-pin and internal-function bodies. Code
identifiers (test filenames like `mq3_multi_class_e2e.rs`, function
names like `iter18e_drop_iterative_default_preserves_hashes`) stay
verbatim per the user's "code identifiers stay verbatim" rule.

Tests: design_index_pin 5/5 + docs_honesty_pin 5/5; workspace builds
clean; full `cargo test --workspace` previously green (every
`test result: ok` line, no FAILED line).

design/contracts/data-model.md

@@ -41,7 +41,7 @@ narrative — defaults, superclasses, diagnostics — lives in
   "params": ["<id>"...],   // names bound in body, in type.params order
   "body": Term,
   "doc": "<optional string>",
-  "export": "<optional C symbol>", // omitted when absent (hash-stable when omitted); see §"Embedding ABI"
+  "export": "<optional C symbol>", // omitted when absent (hash-stable when omitted); embedding-ABI surface — see prose below
   "suppress": [Suppress...] // omitted when empty
 }
 
@@ -65,11 +65,11 @@ narrative — defaults, superclasses, diagnostics — lives in
   "drop-iterative": true   // opt-in; omitted when false (hash-stable when omitted)
 }
 
-// class (typeclass declaration; see Decision 11)
+// class (typeclass declaration; narrative in contracts/typeclasses.md)
 { "kind": "class",
   "name": "<id>",          // class name (e.g. "Show")
   "param": "<id>",          // single class parameter, kind *
-  "superclass": null,       // or { "class": "<id>", "type": "<param>" } — "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
+  "superclass": null,       // or { "class": "<id>", "type": "<param>" } — "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module)
   "methods": [
     { "name": "<id>",
       "type": Type,         // FnSig over the class param
@@ -80,9 +80,9 @@ narrative — defaults, superclasses, diagnostics — lives in
   "doc": "<optional string>"
 }
 
-// instance (typeclass instance; see Decision 11)
+// instance (typeclass instance; narrative in contracts/typeclasses.md)
 { "kind": "instance",
-  "class": "<id>",          // class being instantiated; canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
+  "class": "<id>",          // class being instantiated; canonical form (bare for same-module, "<module>.<Class>" for cross-module)
   "type": Type,             // concrete type expression (never the class param)
   "methods": [
     { "name": "<id>", "body": Term }
@@ -215,13 +215,14 @@ metadata is defined and gated there as well.
 ```jsonc
 // Type-constructor application. `args` omitted when empty
 // (hash-stable when omitted, for non-parameterised cases like Int, Bool, ...).
-{ "k": "con", "name": "<id>", "args": [Type...] }   // "name": canonical form (bare for same-module / primitives, "<module>.<TypeName>" for cross-module; see §"Type::Con name scoping" / ct.1)
+{ "k": "con", "name": "<id>", "args": [Type...] }   // "name": canonical form (bare for same-module / primitives, "<module>.<TypeName>" for cross-module)
 
-// Function type. Decision 10 added paramModes/retMode as
-// metadata on Type::Fn — they are NOT separate Type variants, so every
-// existing match-arm in the typechecker (unify, occurs, apply) keeps
-// working. `paramModes` omitted when every entry is "implicit";
-// `retMode` omitted when "implicit" (hash-stable when omitted).
+// Function type. paramModes/retMode are metadata on Type::Fn —
+// they are NOT separate Type variants, so every existing match-arm
+// in the typechecker (unify, occurs, apply) keeps working.
+// `paramModes` omitted when every entry is "implicit"; `retMode`
+// omitted when "implicit" (hash-stable when omitted). Full mode
+// contract lives in contracts/memory-model.md.
 { "k": "fn",
   "params":     [Type...],
   "paramModes": [ParamMode...],
@@ -232,14 +233,16 @@ metadata is defined and gated there as well.
 { "k": "var", "name": "<id>" }
 
 // Top-level polymorphism only. `constraints` carries class
-// constraints (Decision 11); omitted when empty (hash-stable when omitted).
+// constraints (narrative in contracts/typeclasses.md); omitted when
+// empty (hash-stable when omitted).
 { "k": "forall",
   "vars": ["<id>"...],
-  "constraints": [{ "class": "<id>", "type": "<id>" }, ...],   // "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
+  "constraints": [{ "class": "<id>", "type": "<id>" }, ...],   // "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module)
   "body": Type }
 ```
 
-**`ParamMode`** (Decision 10):
+**`ParamMode`** (full contract in
+[memory model](memory-model.md)):
 
 ```
 "implicit"   — unannotated / back-compat. Treated as `own` by the typechecker.