fix(linker): richer diagnostic for subprocess spawn failures (Windows long-cmdline / long-path)

[ghaith]

Note: this description was drafted by Claude (via Claude Code) and posted on my behalf. Verify before merging. — Ghaith

Summary

Refs #1721 — diagnostic step only, not the mitigation.

When the linker subprocess fails to start, the current diagnostic is the flat An error occurred during linking: {io::Error} (E077). For Windows hosts hitting os error 206 (ERROR_FILENAME_EXCED_RANGE) — the user-reported symptom in #1721 — that message is too terse to be actionable, and 206 covers two distinct conditions that can't be disambiguated from the bare error code:

1. CreateProcess rejecting an assembled command line longer than 32,767 bytes (or 8,191 if going through cmd.exe).
2. CreateFile rejecting a single path longer than MAX_PATH (260 chars) that is not prefixed with \\?\.

This PR adds a small diagnose_spawn_error helper that wraps the failing Command::status() with structured context. It does not change the LinkerError schema (still Link(String) → E077), nor does it ship the actual mitigation (response file, \\?\ prefix). The mitigation will land separately once field reports show which failure mode dominates — see #1721 for the two-step plan.

What this PR does

• New free function diagnose_spawn_error(err: &io::Error, linker: &Path, args: &[String]) -> LinkerError in src/linker.rs.
• The ? on Command::new(...).args(...).status() in CcLinker::finalize is replaced with explicit error mapping that funnels through the helper.
• On any host: the diagnostic now includes the linker path, arg count, approximate command-line length, and the longest individual argument with its length. The "approximate" qualifier is honest — we don't reimplement CommandLineToArgvW quoting; the figure is meant to give the user a sense of scale.
• On Windows specifically when err.raw_os_error() == Some(206): appends a section listing the two possible causes (cmdline > 32K vs single path > MAX_PATH) and two user-side workarounds (move workspace closer to drive root, enable LongPathsEnabled).

Sample output

Linux, generic PermissionDenied (snapshot-pinned by the new test):

An error occurred during linking: permission denied
Diagnostic context:
  - linker:          /usr/bin/ld.lld
  - args:            3
  - cmdline length:  19 bytes
  - longest arg:     12 bytes ('longer-arg-2')

Windows, os error 206 (snapshot-pinned by the #[cfg(windows)] test):

An error occurred during linking: [os-error 206 (locale-dependent)]
Diagnostic context:
  - linker:          ld.lld.exe
  - args:            2
  - cmdline length:  7 bytes
  - longest arg:     3 bytes ('one')
Possible causes on Windows (ERROR_FILENAME_EXCED_RANGE):
  - The assembled command line exceeded 32,767 bytes (CreateProcess limit).
  - A single argument exceeded MAX_PATH (260 chars) without the \\?\ prefix.
Possible workarounds:
  - Move the workspace closer to the drive root to shorten paths.
  - Enable Windows long paths (LongPathsEnabled registry / process manifest).

The Windows test uses an insta filter on the locale-dependent first line because the underlying io::Error display is localized by FormatMessage.

Tests

• spawn_diagnostic_for_generic_io_error — synthetic PermissionDenied + 3 args; pins the generic message shape.
• spawn_diagnostic_for_os_error_206_includes_windows_section (#[cfg(windows)]) — synthetic from_raw_os_error(206); pins the full Windows-specific message via a locale-tolerant filter.

The helper is a pure function over (io::Error, &Path, &[String]), so the tests don't spawn any subprocess.

Out of scope

• Mitigation (response-file linking, \\?\ extended-path prefix) — Step 2 of Windows: linker fails with "os error 206" when build paths or command line exceed Windows limits #1721.
• Pre-flight existence checks on linker inputs (objects, libs, sysroot) — possible follow-up; project-level path validation already lives separately under Validate if paths in include_path exist #1215 / PR fix(project): validate file/include paths and reject empty input sets #1713.
• Capturing linker stderr — today it inherits the parent terminal, which is fine; users see the linker's own diagnostics directly.

[github-actions]

Build Artifacts

🐧 Linux

Artifact	Link	Size
deb-x86_64	Download	38.5 MB
schema	Download	0.0 MB
stdlib	Download	32.4 MB
plc-x86_64	Download	43.6 MB
deb-aarch64	Download	30.9 MB
plc-aarch64	Download	43.5 MB

From workflow run

🪟 Windows

Artifact	Link	Size
stdlib.lib	Download	4.3 MB
stdlib.dll	Download	0.1 MB
plc.exe	Download	38.4 MB

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