P1: JSON-bridge int64 fidelity + schema-text round-trip (Stage 5)

[phenomenon0]

Stage 5 — P1 JSON-bridge fidelity + schema-text round-trip

Branches off main. Go-only. Closes the remaining P1 gaps from the parity roadmap (the schema-hash ship-blocker is handled separately in #5 feat/p1-schema-hash).

JSON bridge (json_bridge.go)

• int64 precision (the cited json_bridge.go:213 bug): ToJSONLoose of an Int GValue now emits a full integer literal via json.Number instead of float64(v.intVal), so values above 2^53 no longer silently round on emit.
• Extended-marker collision-safety: the $glyph key is reserved in extended mode. Emitting a map/struct key or sum tag named $glyph is now a loud error rather than a silently ambiguous marker; on decode, only exactly-shaped marker objects are accepted (extra/missing companion keys are rejected).
• Documented the maintainer decision that the Loose layer intentionally collapses to JSON-like semantics (time/id/bytes → string, struct → object with TypeName dropped, sum → {tag:value}). Typed round-trip is the canonical Parse/Emit path's job only.

Deliberate deviation from the roadmap (surfaced, not blended)

The roadmap asked FromJSONLoose to use json.Decoder.UseNumber so large ints "survive" on input. That conflicts with:

1. the maintainer "Loose = JSON-like collapse" decision, and
2. the cross-impl parity gate — goldens 016_large_int_like / 035_safe_int_boundary require 2^53+1 to render as the float 9.007199254740992e+15, matching JS number (which cannot hold such integers).

So input keeps JSON-like float64 collapse; only the emit direction preserves int64. This is documented in the code and is the correct reconciliation per "surface conflicts, pick the more recent/tested."

Schema text (token.go, parse.go, schema.go)

• Lexer now tokenizes the .. range operator so [0..10] lexes (a lone . is still a lex error). The Range constraint String() no longer double-brackets, so it round-trips as [min..max].
• EmitSchema/ParseSchema now round-trip the packed-decoding-relevant subset: @pack/@tab/@open type flags and per-field @fid(N), @k(wire), @codec(name), @keepnull, and scalar @default(...). (Previously @open was emitted but the parser rejected it.)
• ParseSchema(EmitSchema(s)) is now a fixpoint for that subset — the test asserts text + hash stability and per-field survival.

Remainder (explicitly de-scoped, noted per roadmap)

Container/bytes @default values and the min-len/max-len/regex/enum/unique constraints emit but are not yet parsed back from schema text; they still round-trip through the programmatic schema API.

Verification

ulimit -v 6000000; go test -C go ./glyph/ -skip 'Industrial|Cliff|Perf|Savings|Benchmark' -count=1 → green, incl. TestCrossImpl_* parity. go vet + go build ./... clean.

New tests: json_bridge_fidelity_test.go, schema_text_roundtrip_test.go.

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