perf(wasm): skip empty metadata probes on cell writes

[chh-ay]

Scope

Faster bulk writes in both engines. Part of #59. Stacked on #73 (base perf/core-gaps).

Problem

Profiles of 1,000,000-row ingestion showed every written cell paying hash probes into the sheet's formula and spill maps, even when a sheet has no formulas and no spills, which is the usual case for loaded data.

Change

• SheetData::remove_formula removes a cell's formula and skips the hash when the sheet has none. Every write path in store.rs uses it.
• prepare_cell_write returns at once when the sheet has no spill metadata.
• setBlockPacked skips per-cell source lookups for blocks that carry no formula sources.

Measurement

Both locks held, one pinned CPU, each side loading its own built package; outputs identical.

Workload	Before	After	Change
1,000,000-row mixed-column ingestion	1,311.7 ms	850.0 ms	−35.2%
Window read, sort, filter, distinct, aggregate (untouched)			−0.6% to +2.3%

A/A control: 9 rounds, paired noise within ±3.4%. ABBA: 9 rounds. Centralizing the guard in one helper was checked against the first version: +2.7% (within ±3%).

Size

Binary	Before	After	Change
Default raw	793,255	793,973	+718 (+0.09%)
Default Brotli q11	238,012	237,888	−124
Full raw	1,076,283	1,077,002	+719

Verification

• Rust 147 (default) and 223 (--features analysis) pass; library clippy passes in both configurations.
• Build, docs:check, api:check, typecheck, lint pass; packages 869 pass / 6 skip / 0 fail.

[chh-ay]

Landed on develop in the 0.5.0 release squash: #112 (55cfa82).