feat(csv): make parseLine the synchronous primitive (refs #3765)

[MukundaKatta]

Summary

• Make parseLine the actual internal CSV primitive that both parse() and CsvParseStream build on, addressing the design feedback from feat(csv): add parseLine() convenience for single-line CSV records (refs #3765) #7114 (closed) and aligning with suggestion: investigate simpler CSV-parsing APIs #3765's intent.
• Drop the duplicate field-parsing loop that previously lived inside Parser.#parseRecord in parse.ts — both parse() (sync) and the streaming path now share one set of field/quote rules.
• Public parseLine(line, options) -> string[] is the simple shape suggestion: investigate simpler CSV-parsing APIs #3765 asked for, with BOM strip and trailing CR/LF/CRLF normalization.

What changed

• csv/_io.ts: new sync parseLine carries the whole field-parsing state machine (separator, quotes, escapes, lazyQuotes, comment, trim). The existing async parseRecord becomes a small wrapper that pulls more lines from the LineReader and re-calls parseLine until a record completes. Error column tracking maps absolute positions in the joined input back to (line, column) so multi-line quoted records still report the right line.
• csv/parse.ts: drop Parser.#parseRecord's duplicate field loop; Parser now defers to parseLine from _io.ts. Add the public parseLine export with a clean (line, options) signature.
• csv/parse_test.ts: 12 new tests pin parseLine behavior (happy path, custom separator, escaped quotes, BOM, trailing newline, multi-line quoted body, lazyQuotes, comment, unclosed-field error).

Test plan

• All 133 existing parse + parse_stream steps still pass (145 total with the new parseLine tests).
• Existing error-message assertions (StartLine1, StartLine2, ParseErrorLine, OddQuotes, etc.) preserved with no changes to test expectations.
• Reviewer to confirm parseLine's public surface matches suggestion: investigate simpler CSV-parsing APIs #3765's spirit and that the (line, options) shape is what was wanted.

cc @bartlomieju — this replaces #7114 with the design you sketched in the review there.

[codecov]

Codecov Report

❌ Patch coverage is 91.96429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.61%. Comparing base (cd03740) to head (ccb627d).

Files with missing lines	Patch %	Lines
csv/parse.ts	82.60%	6 Missing and 2 partials ⚠️
csv/_io.ts	98.48%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7118   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files         634      634           
  Lines       51799    51769   -30     
  Branches     9329     9327    -2     
=======================================
- Hits        49009    48982   -27     
+ Misses       2216     2211    -5     
- Partials      574      576    +2     

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[fibibot]

@std/csv is v1.0.6 and csv/mod.ts:184 re-exports everything from ./parse.ts, so the new parseLine lands on the stable surface. Per .github/CONTRIBUTING.md, new public APIs in v1.0.0+ packages need to live in csv/unstable_parse_line.ts, must not be re-exported from mod.ts, and must carry @experimental **UNSTABLE**: New API, yet to be vetted.. Title would shift to feat(csv/unstable): add parseLine.

The underlying refactor — unifying Parser.#parseRecord and the streaming parseRecord onto one field-state machine in _io.ts — is fine; the locate() helper correctly recomputes (line, col) for embedded \n and the EOF branches match the old line.length === 0 / line.length > 0 split.

• nit: parse.ts:14 was previously a \uXXXX-escaped BOM constant; this PR replaces it with the raw U+FEFF character, which renders invisibly in most editors and breaks grep. The new BOM test in parse_test.ts does the same. Please keep the escape form — parse_test.ts already defines a BYTE_ORDER_MARK constant using it at the top of the file.