chain: fix IEEE-754 float noise in tool JSON output (#1)

[kiki830621]

Single-root IDD chain for #1.

Root cause (corrected from the issue's premise)

The issue proposed rounding lat/lon to ~6 decimals. That is a no-op: 25.04 has no exact IEEE-754 Double representation, so (d * 1e6).rounded() / 1e6 lands on the same bit pattern and JSONSerialization still emits 25.039999999999999. The noise lives in the 17-significant-digit Double formatter, not the value — and it affects every non-exact Double this codebase re-serializes (coordinates, speeds, fares), not just lat/lon.

Fix

New JSONSanitize.clean(_:) recursively rewrites every Double to NSDecimalNumber(string: d.description) — the shortest string that round-trips — wired into all 8 sites that re-serialize codebase-constructed dicts (6 jsonResult helpers + 2 inline RailTools sites). Centralizing below the call sites also cleans coordinates riding inside payloads the issue didn't name (e.g. rail_search_stations matches).

Guarantees (empirically verified across the full Double domain):

• Value-preserving — round-trips numerically equal; bit-exact for every finite value except -0.0 (renders 0; JSONSerialization already drops the sign on round-trip regardless).
• Int/Bool untouched — no NSNumber/Float in the dicts (all reals are Double); native scalars don't match as Double (confirmed via NSNumber.objCType).
• Non-finite pass-through — inf/nan rejected by JSONSerialization exactly as before.
• Extreme-magnitude fallback — values beyond NSDecimalNumber's ±128 exponent ceiling fall back to raw formatting = no regression (never occur in transport data).
• Only normal-value change: integer-valued doubles render 25 not 25.0 (identical JSON number, round-trips to 25.0).

Scope — raw TDX passthroughs correctly exempt

Four sites forward TDX's original response bytes verbatim (no deserialize→Double→reserialize round-trip), so they introduce none of our IEEE-754 noise and are intentionally NOT routed through the sanitizer (routing them would require parsing TDX JSON, re-introducing the very problem):

• RailTools.swift:199 rail_find_trains
• RailTools.swift:223 rail_status_train
• RailTools.swift:251 rail_status_station
• MaritimeTools.swift:103-106 maritime_status_schedule

Verification

• TDD: flipped testBusStatusPositionsAssemblesPositions RED→GREEN (asserts clean "lat":25.04).
• JSONSanitizeTests: numerically-exact round-trip across 13 adversarial values (incl. 1e-7, 1e300, Double.greatestFiniteMagnitude, -0.0), Int/Bool preservation, recursion, non-finite, strings/null.
• Cross-module guard: no-noise + clean center-echo assertion in bike nearby.
• Full suite GREEN: 111 tests, 2 keychain-skipped, 0 failures.
• Independent adversarial review (2 reviewers, correctness + completeness lenses, each built & probed): both PASS, no blockers/majors. Surfaced minors (committed-test value set, doc precision, raw-passthrough disclosure) — all addressed in this revision.

Sister observation (out of scope, not chained): the 6 jsonResult helpers are byte-identical — a DRY follow-up could centralize them.

Refs #1

Chain stops at verified — run /idd-close #1 to close after review.