Keep blanks after [ in link text

[bobzhang]

Split out of #152. Found by differential fuzzing against markdown-it-py (commonmark preset).

Bug: the last inline pass strips the leading blanks of each line of a paragraph. For link text it also stripped the blanks after the opening [, which are not at the start of a line.

Repro: [ foo](/u) gave <a href="/u">foo</a>. Expected <a href="/u"> foo</a>. Same for image alt text (![ baz](/i)).

Fix: last_pass and parse_tokens take an optional strip_first (default true). try_link passes strip_first=false for the link text. Blanks that start the following lines are still stripped.

Spec examples: none (the spec suite passes either way).

Tests: src/cmark_html/link_text_leading_blanks_test.mbt.

Depends on: nothing, independent of the other PRs.

[bobzhang]

Automated review: Codex CLI (codex exec, model reasoning effort high, read-only sandbox), 1 round, on git diff against the base branch.

Verdict: signed off, no changes requested.

I sign off on this PR. No concrete correctness, regression, or code-quality problems found.

The change preserves blanks inside link text while retaining the required trimming around line breaks. Tests cover inline links, reference links, image alt text, and continuation-line indentation.

Codex could not run moon test in its read-only sandbox. The tests were run separately on native, js and wasm-gc and pass.