feat(parquet): add WithDictionaryCostFallback writer property

[tom-eon]

Rationale for this change

The pre-first-page dictionary cost check (checkDictionarySizeLimit, mirroring parquet-mr's FallbackValuesWriter.shouldFallBack) compares uncompressed sizes: the dictionary is discarded when dictSize + encodedSize >= rawSize. With a page compressor configured this can discard dictionaries that clearly win after compression — bit-packed dictionary indices are near-incompressible but small, while PLAIN-encoded values often compress far less than the raw-size comparison assumes. In our benchmarks (ZSTD level 3), a mid-cardinality Time64 column written PLAIN after the fallback was 2× larger than the same column dictionary-encoded (7.1 MB vs 3.6 MB compressed); mid-cardinality integer columns lost 20%+.

Callers that decide dictionary usage from their own measurements (e.g. by trial-encoding a sample of the actual data with the actual codec) currently have no way to make an explicit WithDictionaryFor(col, true) stick — the heuristic silently overrides it.

Alternative considered: making the check itself compression-aware — e.g. trial-compressing both branches of the first data page and keeping the winner, or letting callers supply per-column compressibility estimates for the comparison. That fixes the misprediction for everyone rather than only for callers who opt out, but it costs an extra encode+compress of the first page per column (or a new estimate-supplying API surface), needs a guard band to avoid paying it on clear-cut cases, and still guesses for callers who already measured. The opt-out is minimal, preserves the default exactly, and doesn't preclude a compression-aware check later — if there's interest in that direction I'm happy to discuss it in a follow-up.

What changes are included in this PR?

A new per-column writer property following the existing global/per-column property pattern (like WithDictionaryDefault / WithDictionaryFor):

• parquet.WithDictionaryCostFallback(enabled bool) — sets the default for all columns (stored on ColumnProperties, default true, existing behavior unchanged);
• parquet.WithDictionaryCostFallbackFor(path string, enabled bool) / WithDictionaryCostFallbackPath(path ColumnPath, enabled bool) — per-column override;
• WriterProperties.DictionaryCostFallbackEnabledFor(path string) bool — resolution, per-column entry falling back to the default.

When disabled for a column, the pre-first-page cost-based fallback is skipped for that column, so a requested dictionary is kept; the DictionaryPageSizeLimit overflow fallback remains in effect regardless of the setting. The per-column granularity matters for callers that measure encodings per column: columns they measured keep their explicit choice, while unmeasured columns retain the heuristic as a safety net.

Are these changes tested?

Yes — new tests in parquet/file/dict_cost_fallback_test.go covering: (1) default behavior still discards a non-paying dictionary, (2) with the option disabled an explicitly requested dictionary is kept, (3) a per-column disable keeps that column's dictionary while (4) a disable scoped to a different column leaves the fallback active, and (5) the DictionaryPageSizeLimit fallback still applies when the cost fallback is disabled. Existing parquet/file and parquet/internal/encoding suites pass.

Are there any user-facing changes?

New opt-in writer properties; default behavior is unchanged.