fix(build): unblock tinyvec 1.13.0 resolution

[developer0hye]

Summary

• explicitly enable tinyvec's std feature in the published office2pdf manifest
• use Cargo feature unification to avoid tinyvec 1.13.0's broken no_std + alloc code path
• keep the workaround bounded and documented for removal after a patched upstream release

tinyvec is already in the dependency graph, and office2pdf is std-only, so this changes feature selection without adding a new compiled package. The permanent upstream fix is Lokathor/tinyvec#226 for Lokathor/tinyvec#225; that PR is independently validated but has not been merged or released yet.

Related issue

Fixes #1524

Failure reproduced first

A fresh main worktree with no Cargo.lock failed as expected:

$ CARGO_TARGET_DIR=/private/tmp/office2pdf-1524-before-target cargo check --workspace
error: cannot find macro `vec` in this scope
  --> tinyvec-1.13.0/src/tinyvec.rs:710:21
error: could not compile `tinyvec` (lib) due to 1 previous error

Testing

• CARGO_TARGET_DIR=/private/tmp/office2pdf-1524-before-target cargo check --workspace
• CARGO_TARGET_DIR=/private/tmp/office2pdf-1524-before-target cargo test --workspace
• CARGO_TARGET_DIR=/private/tmp/office2pdf-1524-before-target cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• CARGO_TARGET_DIR=/private/tmp/office2pdf-tinyvec-std-wasm-target cargo check --target wasm32-unknown-unknown -p office2pdf
• CARGO_TARGET_DIR=/private/tmp/office2pdf-tinyvec-std-wasm-target cargo check --target wasm32-unknown-unknown -p office2pdf --features wasm
• CARGO_TARGET_DIR=/private/tmp/office2pdf-tinyvec-std-wasm-target cargo check --target wasm32-unknown-unknown -p office2pdf --features wasm-cjk-font
• CARGO_TARGET_DIR=/private/tmp/office2pdf-1524-publish-target cargo publish --dry-run -p office2pdf --allow-dirty
• packaged normalized manifest contains tinyvec = "1", features = ["std"], and default-features = false
• mandatory documentation freshness audit: PASS:

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: This only changes Cargo feature selection for an existing transitive dependency; parser, IR, rendering, and fixtures are unchanged.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining visual deviations each reference an open issue (none; no rendered change)

[developer0hye]

Closing unmerged because upstream tinyvec 1.13.1 is now published with the qualified alloc::vec! fix. A fresh, unmodified office2pdf main worktree with no Cargo.lock resolved tinyvec 1.13.1 and completed cargo check --workspace successfully. The temporary feature-unification dependency is therefore no longer needed. Existing affected PR runs will be rerun against the fixed release.