Compile the Inno Setup scripts in CI so installer breakage is caught before release #17

Problem

The Inno Setup scripts in installer/ are never compiled by cargo, so a change that breaks them passes every check in CI and only fails at release time.

This is not hypothetical. In PR #16 both installer/openwritr.iss and installer/openwritr-x64.iss stopped compiling:

Error on line 114 in installer/openwritr.iss: Column 65:
'BEGIN' expected.
Compile aborted.        (ISCC exit 2)

Cause: a new [Code] block used a Pascal { ... } comment whose text contained the Inno constant {userstartup}. Pascal block comments do not nest, so that } closed the comment early and the remaining prose was parsed as code. cargo fmt, cargo clippy and cargo test were all green throughout — the breakage was only caught by manually running ISCC.exe.

Proposal

Add a fast installer-syntax job to CI that compiles both scripts and fails the build on a non-zero ISCC exit code.

Implementation notes

• Needs a Windows runner with Inno Setup 6. It is available via choco install innosetup or winget install JRSoftware.InnoSetup on windows-latest.
• A full compile needs the payload the scripts reference, which we do not want to build just for a syntax check:
  • ..\target\stage\{arm64,x64} (created by scripts/prepare_release.py)
  • ..\LICENSE and installer\openwritr.ico
• Cheapest reliable approach: stage a throwaway tree with the real LICENSE and .ico plus a dummy openwritr.exe in target/stage/<arch>/, then run ISCC.exe /Qp on each script and assert exit 0. This is exactly how the PR Add "Run at startup" toggle in Settings #16 breakage was reproduced and then confirmed fixed, and it takes seconds.
• Write the output to a scratch directory, not target/dist, and do not upload the resulting setup executables as artifacts — they contain a dummy payload and must never be mistaken for a release build.
• Run the job only when installer/** changes, to keep it off the hot path for ordinary code PRs.

Acceptance criteria

• CI compiles both installer/openwritr.iss and installer/openwritr-x64.iss and fails on a non-zero ISCC exit code.
• Reintroducing the PR Add "Run at startup" toggle in Settings #16 { ... } comment bug makes the job fail.
• The job does not require a full release build and does not publish its output.
• The job is skipped when no installer file changed.