Fix build abort on broken autodoc reference blocking link resolution

[aether-png]

Fixes #475

Root cause

build_mdx_files() aborted the entire build immediately on the first file with a broken [[autodoc]] reference. This happened before resolve_links() ran, so every [~Class.method]-style reference across the whole doc set was left unresolved when any single reference broke — including the trailing-underscore case in #475, where the unresolved raw text got picked up by markdown's emphasis parsing.

A second issue surfaced once the abort was removed: check_toc_integrity(), called right after, independently failed because the file that errored was legitimately absent from the output directory (never written) but still listed in _toctree.yml.

Fix
build_mdx_files() now collects per-file errors and continues instead of raising immediately
check_toc_integrity() accepts a set of known-failed files and skips them when checking for missing output
build_doc() now lets resolve_links, notebook building, and toctree renaming run regardless of MDX errors, and raises the deployment-failure error only at the very end
Testing
Reproduced the original bug: single broken [[autodoc]] reference → full build abort → every reference left unresolved
Verified fix: same broken reference → build completes, clip_grad_value_ (and all other references) resolve correctly, error is raised only at the end
Verified multi-error case: two broken references in different files → both correctly reported together, neither dropped
Verified clean build (no broken references) has zero regressions
Ran full existing test suite; confirmed the 3 pre-existing failures (GlmAsrForConditionalGeneration import error, a cascading AutoProcessor NameError, and a Windows-specific [WinError 2] subprocess issue in test_format_code_example) are present identically on unmodified main, unrelated to this change