feat: cache date and number formatters

[Duansg]

Purpose of the pull request

DateUtils and NumberUtils rebuilt a formatter on every cell. This PR caches them per thread, keyed by locale + pattern, so a formatter is built once and reused.

The java.util.Date path in the very same class was already cached; the java.time and DecimalFormat paths just never got the same treatment. This PR closes that gap.

What's changed?

Both caches are ThreadLocal (not a shared static map) so they are bounded by thread lifetime and released by the existing cleanup, consistent with the sibling DATE_FORMAT_THREAD_LOCAL.

1. DateUtils — new DATE_TIME_FORMATTER_THREAD_LOCAL cache; all 8 sites resolve through getCacheDateTimeFormat(pattern, locale). Cleared in the existing removeThreadLocalCache().
2. NumberUtils — new DECIMAL_FORMAT_THREAD_LOCAL cache via getCacheDecimalFormat(pattern, roundingMode), plus a new removeThreadLocalCache() wired into the read/write finish paths (WriteContextImpl, ExcelAnalyserImpl).

Benchmark

Per cell (JMH 1.37, -prof gc, 2 forks; allocation deterministic ±0.001 B/op)

┌──────────────┬─────────────┬────────────┬─────────────┬──────────────┬─────────────┬─────────┐
│  Benchmark   │ Time before │ Time after │   Time Δ    │ Alloc before │ Alloc after │ Alloc Δ │
├──────────────┼─────────────┼────────────┼─────────────┼──────────────┼─────────────┼─────────┤
│ dateFormat   │    422.8 ns │   205.4 ns │        −51% │        776 B │       396 B │    −49% │
├──────────────┼─────────────┼────────────┼─────────────┼──────────────┼─────────────┼─────────┤
│ dateParse    │    680.0 ns │   631.1 ns │        −7%  │       1272 B │       880 B │    −31% │
├──────────────┼─────────────┼────────────┼─────────────┼──────────────┼─────────────┼─────────┤
│ numberFormat │    905.2 ns │   749.4 ns │        −17% │       1656 B │       480 B │    −71% │
├──────────────┼─────────────┼────────────┼─────────────┼──────────────┼─────────────┼─────────┤
│ numberParse  │    492.8 ns │   277.0 ns │        −44% │       1568 B │       376 B │    −76% │
└──────────────┴─────────────┴────────────┴─────────────┴──────────────┴─────────────┴─────────┘

End-to-end (500000-row CSV, median of 3)

┌───────┬───────────────────┬──────────┬──────────┬──────┐
│ Phase │      Metric       │  Before  │  After   │  Δ   │
├───────┼───────────────────┼──────────┼──────────┼──────┤
│ Write │ Thread allocation │ 2,386 MB │ 1,593 MB │ −33% │
├───────┼───────────────────┼──────────┼──────────┼──────┤
│ Read  │ Thread allocation │ 1,960 MB │ 1,279 MB │ −35% │
└───────┴───────────────────┴──────────┴──────────┴──────┘

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

[Copilot]

Pull request overview

This PR improves formatting performance by caching date/time and numeric formatter instances in thread-local maps keyed by locale + pattern, avoiding per-cell formatter construction and ensuring caches are cleared at the end of read/write lifecycles.

Changes:

• Added a ThreadLocal<Map<String, DateTimeFormatter>> cache in DateUtils and routed java.time parse/format through a cached getCacheDateTimeFormat(...).
• Added a ThreadLocal<Map<String, DecimalFormat>> cache in NumberUtils, with cleanup wired into both write and read finish paths.
• Expanded unit tests to validate cache behavior across default FORMAT locale changes and to verify thread-local cleanup.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
fesod-sheet/src/main/java/org/apache/fesod/sheet/util/DateUtils.java	Adds a per-thread DateTimeFormatter cache and clears it in removeThreadLocalCache().
fesod-sheet/src/main/java/org/apache/fesod/sheet/util/NumberUtils.java	Adds a per-thread DecimalFormat cache keyed by default FORMAT locale + pattern, plus public cache cleanup.
fesod-sheet/src/main/java/org/apache/fesod/sheet/context/WriteContextImpl.java	Calls NumberUtils.removeThreadLocalCache() during write finish cleanup.
fesod-sheet/src/main/java/org/apache/fesod/sheet/analysis/ExcelAnalyserImpl.java	Calls NumberUtils.removeThreadLocalCache() during read finish cleanup.
fesod-sheet/src/test/java/org/apache/fesod/sheet/util/DateUtilsTest.java	Adds tests covering locale-sensitive DateTimeFormatter caching and cleanup verification.
fesod-sheet/src/test/java/org/apache/fesod/sheet/util/NumberUtilsTest.java	Adds tests covering locale-sensitive DecimalFormat caching and cleanup across read/write finishes.

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