Fix blob column handling for binary data with embedded nulls

[carpentry-agent]

Summary

The sql_blob case in SQLiteColumn.to-carp was using from-text (backed by sqlite3_column_text/SQLiteColumn_from_str), which treats data as a null-terminated C string. Binary blobs containing embedded null bytes were silently truncated.

Changes:

• Type.Blob now wraps (Array Byte) instead of String, preserving exact blob length
• Added blob_len field to SQLiteColumn struct to carry the byte count through the C layer
• Added SQLiteColumn_from_blob to convert blob data into a length-aware Carp Array
• Fixed sqlite3_bind_blob call to use blob_len instead of strlen, which also truncated on bind
• Added test: blob with embedded nulls ([0b 1b 0b 2b 3b]) round-trips correctly

Breaking change: Type.Blob now holds (Array Byte) instead of String. Code constructing or pattern-matching on Blob values needs updating.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Build: compiles cleanly on ARM Linux.
Tests: all 26 tests pass, including the new blob round-trip test ([0b 1b 0b 2b 3b] with embedded nulls).
CI: passing on both macos-latest and ubuntu-latest.

Findings

None. The fix is correct and addresses the root cause at every layer:

1. Type change (sqlite3.carp:77): Type.Blob now wraps (Array Byte) instead of String — the right representation for binary data.
2. Blob retrieval (sqlite3_helper.h:194–199): uses sqlite3_column_blob + sqlite3_column_bytes instead of sqlite3_column_text. Allocates exactly len bytes (no null terminator) and stores blob_len in the column struct.
3. Blob binding (sqlite3_helper.h:252): uses val.blob_len instead of strlen(val.s) — the critical fix that prevented truncation on write.
4. SQLiteColumn_from_blob (sqlite3_helper.h:70–76): creates a Carp Array from the column's s pointer and blob_len. Ownership transfer is correct — the SQLiteColumn is consumed by value, so no double-free.
5. SQLiteColumn_blob (sqlite3_helper.h:62–68): takes the Carp Array by value, stores a.len and a.data. Again, ownership is clean.

Minor note: blob_len is uninitialized for non-blob column types (int, float, text, null). This is fine since it's only read in the blob code path, but it adds sizeof(int) to every SQLiteColumn. Not a concern in practice.

Breaking change (Blob String → Blob (Array Byte)) is clearly documented in the PR description.

Verdict: merge

Correct fix for a real data-corruption bug, CI passing, well-tested.