Numfmt: TEXT() and chart labels parse each format code once (#806)

[vangel76]

Fixes #806

What was redundant

text_function called NumberFormat::parse(code) on every call, and the evaluator calls it once per element, so =TEXT(A1:A100000,"0.00") parsed the same code 100,000 times. chart/src/resolve.rs did the same per resolved value, and Fmt::new in chart/src/axis.rs per label helper call. A parsed format depends on the code text only (date system and value are applied when formatting), so one parse per distinct code is enough. No timings claimed.

What changed

• numfmt: NumberFormat::parse_cached(code), a per-thread cache (thread_local, no lock, no unsafe, no dependency, wasm-safe). At most 64 codes; a full cache is emptied. Codes over 256 bytes are never stored. If the cache is unavailable (thread shutdown, re-entrant borrow) it falls back to a plain parse; no unwrap/expect.
• text_function, chart/resolve.rs and chart/axis.rs use it. The calc evaluator gets it through text_function. NumberFormat::parse is unchanged.

Tests (crates/numfmt/src/tests.rs)

• Equality: 45 codes (numbers, dates, fractions, text sections, conditions, scientific, fill, invalid codes) x 14 values x both date systems. text_function equals a copy of the old uncached implementation with the cache cold and warm, and format_value with parse_cached equals format_value with parse.
• Counter (cfg(test) parse counter): 10,000 values with one code parse once; two alternating codes parse twice; capacity overflow evicts (old code reparsed, newest still cached); a long code is parsed every time.

Commands run

cargo test -p gridcraft-numfmt, cargo test -p gridcraft-calc -p gridcraft-functions -p gridcraft-chart, cargo test -p gridcraft-engine --lib display, cargo clippy -p gridcraft-numfmt -p gridcraft-chart --all-targets -- -D warnings, cargo fmt on numfmt and chart, cargo xtask layers, cargo check -p gridcraft-numfmt --target wasm32-unknown-unknown.

Left out

• The engine's global Mutex<HashMap> cache in display.rs is untouched (it holds the lock while parsing).
• Per-call allocations in formatting are not part of this PR.
• Overlap with M12: regional formats (typed and displayed numbers, dates and times) #447 checked: its cache is for constant region codes keyed by &'static str; this one is keyed by user code. Possible textual conflict in the lib.rs imports only (mine adds a self-contained module and uses full paths).

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[echelon]

Merged — thank you @vangel76 for the contribution!

I merged main into your branch to resolve conflicts with other PRs that landed first.