perf: borrow instruction stream across unbudgeted tail dispatch

[matthargett]

Draft: standalone tail-dispatch change on next. This PR no longer depends on the closed #63.

Unbudgeted tail dispatch borrows the current function's instruction slice across opcode handlers and retains its Shared<WasmFunction> owner. It rebinds only after a function switch. Fuel/time-budgeted dispatch is unchanged, and no unsafe code is added. A focused test covers linked modules, a host import, indirect calls, returns, and fuel suspension.

Measured against current next (693d590) on the efficiency cores of an iPhone 12 (A14), iPhone XS Max (A12) and iPhone SE (A13); each cell is the change in cycles per call, median of five interleaved launches per build:

benchmark	A14	A12	A13
xmrsplayer (1024-frame buffer)	−6.0%	−11.4%	−9.0%
audio DSP (1000 frames × 512)	−8.9%	−15.1%	−13.7%
graphql-validation (AS)	−7.3%	−9.9%	−8.0%
multi-memory twin: one memory	−7.0%	−11.4%	−11.3%
crc32 (64 KB)	−13.5%	−20.9%	−15.6%
convolution 256×256	−12.6%	−14.0%	−12.3%
sieve (10000)	−11.9%	−14.2%	−12.8%
bulk_memory (memory.copy/fill)	−8.6%	−13.5%	−13.4%
matmul relaxed-simd FMA	−5.9%	−13.1%	−11.6%
GC binary trees (~130K struct.new)	−2.5%	−1.8%	−2.8%
fib(30)	−6.1%	−12.1%	−11.6%
tail-call FSM (65536 return_call)	−3.2%	−1.4%	−5.6%
call_indirect (200K)	−2.5%	+3.8%	−1.3%
call_ref (200K)	+0.4%	+6.2%	−1.4%
vtable_poly4 (200K)	+1.1%	+5.2%	−1.0%
EH parser, exnref (4096 stmts, 25% throw)	+4.4%	+12.5%	+2.9%
geomean, cycles	−5.7%	−7.4%	−8.2%
geomean, instructions	−1.9%	−2.2%	−2.0%
geomean, wall time	−5.9%	−9.9%	−8.3%

note that the call_indirect benchmark regression on A12 (iPhone XS) is something I've seen before with wasmtime/WAMR benchmarking+optimization, and it's due to the efficiency cores on that silicon having much weaker branch prediction. it ended up being a "nice to have" once the relaxed simd128 lowering was optimized, though: under WAMR, xmrsplayer as WASM bundle was able to play a 16-track ScreamTracker3 module without sound skips or thermal throttling. so, in the words of The Guide, Don't Panic :D