🐛 FIX: report accurate source lines for directives and text

[chrisjsewell]

Fixes three classes of wrong source-line attribution, plus two rST-parity alignments.

Directive option warnings point at the offending option

Previously all option warnings pointed at the directive's opening line (:opt: style) or the opening --- (YAML style), and unknown keys were aggregated into one warning:

```{note}
---
class: tip
name: mynote
align: whatever   <- line 7
---
```

reported <string>:4: 'note': Unknown option keys: ['align'] .... Now each warning (unknown key, invalid value, bad YAML, # comments) carries the offending line — <string>:7: 'note': Unknown option key: 'align' ... — for both option styles, resolving the long-standing TODO advance line number:

• parsers/options.py gains a public options_to_tokens (per-key Positions; options_to_items is now a thin wrapper), and its State records comment lines.
• _parse_directive_options computes an options_position per block style and per-key absolute lines; the --- block split is now line-based: text trailing a closing delimiter still becomes body content (as previously), but is kept as its own line, so it — and every body line after it — maps exactly (the old character-based split shifted all subsequent body lines by one).
• Directives synthesized from non-source content (HTML images/admonitions) attribute option warnings to the element's own line.

First-line body content was off by one

For no-argument directives with content on the opening line (```{note} text), every body warning reported one line too low. body_offset is now -1 for the merged first line (documented in DirectiveParsingResult; all internal consumers verified, and myst-nb's code-cell cannot hit this branch since it takes an argument).

Stale document.current_line (closes #546)

run_directive set document.current_line and never reset it, so docutils' Node.setup_child stamped the most recent directive's line (or 0) onto every subsequently-created line-less node — every Text node in the document. This is exactly the "errors attributed to the most recent directive" / "line 0" behaviour reported for sphinxcontrib-spelling, and visible in gettext output (table cells extracted at index.md:0). The renderer now keeps current_line in sync as nodes are created; the regenerated .pot fixtures show cells at their real lines.

rST-parity alignments

• block_text (supersedes Aligns directive's block_text content to rST's #1048, with thanks to @kdeldycke): directives now receive the full directive text — opening fence, options block, unparsed body, closing fence — matching rST, including the header/closing lines that Aligns directive's block_text content to rST's #1048 noted it could not reconstruct. Third-party directives that compute absolute lines from block_text (e.g. click-extra) now get rST-consistent input. Note: erroring directives that embed self.block_text in their error output (list-table, figure, etc.) now show the full fence rather than only the body.
• include with :literal:: the literal block's line is now start-line + 1 (exact docutils behaviour), not a hardcoded 1.

Behaviour changes to be aware of

• Unknown-key warnings changed shape: one per key (Unknown option key: 'x' (allowed: [...])) instead of one aggregate — the myst.directive_option subtype is unchanged, so suppression configs keep working.
• Old-vs-new probes were run against master for: the full docutils rendering suite, a sphinx + sphinx-design project (HTML and searchindex.js byte-identical), myst-nb notebook reading (byte-identical, including malformed metadata blocks), gettext, includes, and a ~30k-input fuzz of the option-parsing rewrite — no rendering differences beyond the intended warning lines/messages and the two rST-parity items above.

Known limitations (documented, out of scope)

• Warnings for inline syntax report the enclosing paragraph's first line (markdown-it inline tokens carry no per-line maps).
• Warnings inside multi-line substitution output can map past the substitution's location.
• Line attribution inside {include}d markdown is off by one (pre-existing, both before and after this PR) — to be tracked separately.

All changed fixture numbers were hand-verified against their source snippets; new conformance cases in reporter_warnings.md lock in the corrected lines (markdown in → warning+line out, reusable as a language-agnostic corpus).

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