fix(flatfiles): reject zero-column and over-width header drift in block decode

[userFRM]

Problem

decode_block in thetadatadx-rs/src/flatfiles/decode.rs passed two drifted/hostile-header shapes silently, unlike the mid-row truncation guard that already fails loud on the same surface:

1. A header declaring fmt_count == 0 over a non-empty DATA block decoded to zero rows with no error. A drifted zero-column header vanished silently.
2. A row carrying more fields than the header's column count was silently clipped (the FIT reader drops surplus fields past the buffer length). The FPSS sibling decode path (fpss/delta.rs) rejects the same width drift.

Solution

Both now return Error::decode_codec, matching the FPSS delta path's width guard and the existing truncation guard:

• n_columns == 0 over a non-empty block returns a zero-column decode error (an empty block still returns Ok with no rows).
• A row with n > n_columns returns an over-width decode error. Delta rows legitimately carry n < n_columns (trailing carry-forward), so only wider rows are rejected.

Regression tests cover both shapes.

Impact

A drifted flat-files header fails loud instead of producing silently wrong output. No behavior change on well-formed blocks; no version bump.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR tightens flat-file FIT block decoding so drifted/hostile headers fail fast instead of silently producing empty output or clipped rows, aligning behavior with the FPSS delta decoding path and the existing mid-row truncation guard.

Changes:

• Rejects non-empty DATA blocks when n_columns == 0 with a typed codec decode error.
• Rejects rows whose decoded field count exceeds the header column count (n > n_columns) to prevent silent truncation.
• Adds regression tests for both drifted-header shapes and documents the behavior change in both changelogs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
thetadatadx-rs/src/flatfiles/decode.rs	Adds explicit guards for zero-column headers over non-empty blocks and over-width rows, plus regression tests.
docs-site/docs/changelog.md	Documents the new “fail loud” behavior for drifted headers in the docs changelog.
CHANGELOG.md	Documents the new “fail loud” behavior for drifted headers in the root changelog.

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