[cscheid]

Closes #944. Braid: bd-ex8qvuwu. Triage record: claude-notes/issue-reports/944/triage.md.

What

An alias whose redirect stub differs from a rendered page only by case (r/index.qmd declaring aliases: [/R/]) was a hard Q-5-25 error with no escape hatch. That implemented decision 3 of the 2026-08-12 aliases plan, whose premise ("no realistic scenario wants a case-only pair") the Posit Connect docs port refuted: the site is served from S3, where R/ and r/ are distinct keys and the old URLs must keep redirecting.

This PR keeps the error as the default and adds an affirmative option:

website:
  case-sensitive-paths: true

With it set:

• plan_alias_stubs compares paths exactly, so a case-only pair plans two stubs. Each stub that would have collided carries a CaseFoldTwin naming the page or earlier stub it folds onto.
• write_alias_redirects probes the output directory (probe file written lowercase, looked up uppercase; only when a twin exists) and, if the directory folds case, skips the twinned stub with the new warning Q-5-51 instead of overwriting the page with a redirect. Pages always win; between two stubs the one that sorts first wins.
• Both Q-5-25 diagnostics now end with a hint naming the option. A non-website project that sets it gets a "no effect" warning, like website.llms-txt.

Docs: Q-5-25.qmd and the aliases guide gain a "hosts that distinguish case" section and an expanded Quarto 1 comparison (Q1 compares exact strings, so it writes both files on Linux and silently replaces the page on macOS/Windows). New Q-5-51.qmd, catalog entry, sidebar entry. Decision 3 of the plan is amended with a pointer to the triage.

Tests — please watch both filesystem kinds in CI

The new integration tests do not gate on cfg!(target_os). Each probes the temp directory it rendered into and asserts the outcome that directory calls for:

• case-sensitive (Linux CI, or a case-sensitive macOS volume): both files written, no Q-5-51, stub redirects to its page;
• case-insensitive (default macOS, Windows): page intact, no stub, Q-5-51 present, no probe file left in _site.

Locally both branches ran: default APFS, and a Case-sensitive APFS disk image mounted as TMPDIR. This is the first test in the repo whose assertions depend on the runner's filesystem, so the Linux and macOS/Windows jobs are exercising different branches of the same tests.

cargo xtask verify --skip-hub-build green locally. cargo clippy -p quarto-core --all-targets -- -D warnings clean. The Windows cross-check (--target x86_64-pc-windows-msvc) cannot run from macOS here because wasmtime/tree-sitter/aws-lc-sys build scripts compile C for the target; the new code uses no platform-specific APIs (SystemRuntime only).

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