Go: the table wire's id-table form (#435)

[gafferongames]

The table wire moved to its id-table form in #507 (docs/SPEC-TABLES.md §3): the form byte, id reference, kind, payload terminated by the zero reference, canonical LEB128 for every length, count, index and reference, sixty-four-bit identity, and the trailing id table located from the end. The C++ reference and the compiler's own engine carry it; this port still writes the previous form.

What moves

• Identity at sixty-four bits. A field's id, an enum variant's, a union arm's and a table's own name id are all fnv1a64(name), no fold and no rebound. The reserved node-table id is 0xFFFFFFFFFFFFFFFF.
• Every number is a canonical LEB128 — length, count, index, id reference — and a non-minimal spelling is malformed. A length cannot be patched in place any more, so every body's length is measured before it rides.
• The id table. Every id the body used, once each, in first-use order over the whole wire; the body names them by 1-based reference, and reference 0 names no id. It is the last thing in the file and a reader finds it from the END: the final eight bytes are the entry count, a fixed little-endian u64.
• The form byte is 1 and is read FIRST. A byte this reader does not know is a REFUSAL by name, never damage, and the read report carries a verdict beside its five counters.
• Three kinds: 30 an enum, carrying the reference to its variant name's id; 31 the escape; 32 the payload-free arm.
• An arm header is a field header and carries the arm's KIND, so a retyped arm is an ordinary kind mismatch.
• The node table rides in ONE field under the reserved id — a 64-bit L frames a numbering of any size.

What to pass

The corpus is pinned in the new form. The five wire-carrying conformance surfaces are ABSENT for this port until the codecs move: wire, report, json-read, json-write and json-hostile — every surface whose expectation is wire bytes. Restoring them means passing, over testdata/conformance/tables/MANIFEST.txt:

• every instance row byte for byte after a save, which holds elision, first-use order and distinctness at once;
• every report row's five counters AND its verdict, including form_zero, form_two and form_ff (the refusal), the escape_* and nopayload_* rows (kinds 31 and 32 at every position), the a2_arm_* rows (the arm's kind byte and its length), and k1_as_k2 / k2_as_k1 (the enum kind against the raw integer);
• tables-<lang>-wire-fuzz against the compiler's engine as the oracle, with the reference-class, id-table, canonical-spelling and form-byte passes §4.2 names.

Part of #349 and #435. PORTING.md carries the cell.

[gafferongames]

Noted by the certification debug (#534, #542): Go's TestLoadAllocatesNothing is green while loading a golden the port now refuses, so it measures the refusal and not the read path. Vacuous until this port wakes to the id-table wire; part of the wake list.