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/authoring-surface.md

@@ -1,6 +1,6 @@
 # Authoring surface
 
-## Decision 6: authoring surface
+## Authoring surface
 
 Form (A) is implemented as
 the `ailang-surface` crate (parser + printer). Form-A is
@@ -22,7 +22,7 @@ one projection among potentially many. Concretely:
   addressed representation of a module. All hashing, content-
   addressing, cross-module references, and typecheck/codegen input
   flow through the JSON-AST. **No new hashable form is introduced.**
-- The textual surface (form A, this Decision) is the **AI authoring
+- The textual surface (form A, this contract) is the **AI authoring
   projection**: optimised for me producing programs token-efficiently
   and for foreign LLMs producing programs from a spec. It is not
   optimised for human authors and does not need to be human-pleasant.
@@ -55,11 +55,11 @@ the AST and remain projection-agnostic.
    shape. The full bijection between `.ail.json` and `.ail` (both
    directions, BLAKE3-stable hashing, Float-bits-hex encoding,
    workspace-CI enforcement points) is anchored as the top-level
-   [Roundtrip Invariant](roundtrip-invariant.md) — this constraint records that Decision
-   6's surface-design choice must satisfy that invariant; the
-   invariant itself lives at top level because the property is
-   load-bearing on the language identity, not on this Decision's
-   surface-design rationale.
+   [Roundtrip Invariant](roundtrip-invariant.md) — this constraint
+   records that the surface-design choice must satisfy that
+   invariant; the invariant itself lives at top level because the
+   property is load-bearing on the language identity, not on this
+   file's surface-design rationale.
 3. **No external symbols.** ASCII only. No Greek (`∀`), no arrows
    (`→`), no subscripts. Reasoning: I substitute mojibake for
    non-ASCII characters under context pressure; foreign LLMs vary