fix(globals): #649 i64.const global initializers reach the emitted image — decoder captures both words, startup materializes the R9 table (#652)

The decoder's `init_i32` captured only a leading `i32.const`; an i64
global's init decoded to `None` and every consumer's `unwrap_or(0)`
silently ZEROED the slot. #645 fixed the get/set PAIR lowering and the
width-aware slot layout, but a nonzero `(global (mut i64) (i64.const X))`
read before any set still returned garbage — the init existed nowhere in
the output.

Decoder: `WasmGlobal.init_i32` is RENAMED to `init: Option<GlobalInit>`
(I32/I64) so no consumer survives by wildcard. f32/f64 inits stay `None`
on purpose — float global access is the GI-FPU-001 (#369/#648) loud-skip
lane and capturing bits here must not quietly unskip it.

Consumers audited (all of them, post-rename):
- `identify_stack_pointer_global`: matches `GlobalInit::I32` only (an SP
  is an i32 address; i64 inits are data, never promoted).
- `--native-pointer-abi` `__synth_globals` region: stays 4-byte i32
  slots; wide globals are already REFUSED up front (#643 bail — the
  decline-loudly arm), so only I32 inits can land. Byte-identical for
  i32-global modules (verified against origin/main).
- Self-contained Cortex-M image (the #649 repro shape): globals were
  never materialized AT ALL — R9 was never even initialized. Now
  `generate_minimal_startup` gets the table words (#643 summed-width
  layout, i64 = two words lo-first), points R9 at
  `0x2000_0000 + linmem_size`, and stores every word before calling the
  user function; RAM sizing + the NoBits region cover the table. Both
  the all-exports and single-function builders. Tables past the STR.W
  #imm12 range decline loudly. No globals => the historical 28-byte
  startup blob and the whole ELF stay BYTE-IDENTICAL (verified).

Red -> green: scripts/repro/i64_global_init_649_differential.py runs the
default cortex-m4f image's REAL reset path in unicorn (artifact-derived,
nothing fabricated), then the stateful export sequence vs one wasmtime
instance. origin/main: FAIL (4 divergences — init reads return vector-
table garbage). This branch: PASS (init reads, i32-after-i64 offset
canary, #645 set-then-get pin, control).

Gates: frozen anchors 10/10, #643 oracle PASS, cargo test --workspace
green, fmt + clippy --workspace -D warnings clean.

Fixes #649. Refs #645, #643, #648, #369.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: avrabe