tidy: clear the milestone fieldtest findings (refusal reasons, --help, ledger + glossary)

Resolves the four non-bug findings from the inferential-validation milestone
fieldtest (the bug — the broken-pipe panic — is fixed in 1088320):
- [friction] generalize refusals now carry a reason instead of a bare usage dump:
  <2 instruments, a multi-value candidate flag, a missing knob, a duplicate
  symbol, and a non-stage1-r strategy each get a one-line why (mirroring the
  R-only metric refusal). Two parser tests strengthened to pin the message.
- [friction] --select <argmax|plateau:mean|plateau:worst> added to the
  walkforward branch of the global USAGE (aura --help) — the plateau objective
  was undiscoverable from the only help the CLI prints (the per-subcommand usage
  already carried it).
- [spec_gap] C1 gains a scope note: bit-identity is per-run; a derived metric
  recomputed by two different command paths may differ by floating-point
  reassociation (<=1 ULP), which is not a C1 violation — ratifies the
  fieldtester's reading of the sweep-vs-generalize sqn difference.
- [spec_gap] the milestone vocabulary is added to docs/glossary.md:
  cross-instrument generalization, deflated score, overfit probability,
  neighbourhood score, plateau selection, sign-agreement, worst-case floor.

The fieldtest triage spec (docs/specs/fieldtest-*) is removed now its findings
are resolved; the tracked fieldtests/ scenario corpus stays.

Verified: cargo test --workspace green (0 failed), clippy --workspace
--all-targets -D warnings clean; `aura --help` shows --select; the refusals
print their reason.

refs #146

docs/design/INDEX.md

@@ -113,6 +113,14 @@ source of truth — not session memory.
 loop that reaches a unique state after each input tick. Same input (incl. seed)
 → bit-identical run. Two backtests are fully disjoint and run concurrently
 without locking.
+**Scope (ratified, fieldtest 0078).** The bit-identity is *per run* — one backtest
+of given (inputs, seed) reproduces byte-for-byte. A *derived metric* recomputed for
+the same params by two *different command paths* (e.g. a swept member's `sqn` vs the
+same cell re-run under `aura generalize`) may differ by floating-point reassociation
+(≤1 ULP), since the two paths accumulate the same logical reduction in a different
+operation order (IEEE-754 non-associativity). This is not a C1 violation: C1 governs
+the determinism of a single run, not the cross-command bit-identity of a re-derived
+statistic.
 **Forbids.** Concurrency *within* a single sim; any nondeterministic input that
 is not captured as an explicit input (see C11, C12).
 **Why.** Real money rides on backtest results; reproducibility and an audit