feat(cache): support ALZ library refs and overwrite flag in cache create

[Copilot]

• Add --library-overwrite-enabled flag wired to AlzLib.Options.AllowOverwrite
• Allow --library to accept ALZ-library references in the form path@ref
• Allow --library to be specified multiple times
• Update flag help text and command long description
• Add unit tests for library reference parsing
• Move parseLibraryReference into the alzlib package as public constructors
  • Add NewAlzLibraryReferenceFromString (inverse of AlzLibraryReference.String())
  • Add NewCustomLibraryReferenceFromString for symmetry
  • Add universal NewLibraryReference with auto-detection
• Use alzlib.NewLibraryReference from the cache create command
• Move parser tests to metadata_test.go and cover the new constructors

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.82759% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.91%. Comparing base (8eb327c) to head (e3a34a9).

Files with missing lines	Patch %	Lines
cmd/alzlibtool/command/cache/create.go	0.00%	30 Missing ⚠️
metadata.go	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
+ Coverage   47.76%   47.91%   +0.14%     
==========================================
  Files          54       54              
  Lines        4865     4913      +48     
==========================================
+ Hits         2324     2354      +30     
- Misses       2255     2272      +17     
- Partials      286      287       +1     

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[matt-FFFFFF]

@copilot fix this minor lint error

  Error: cmd/alzlibtool/command/cache/create.go:282:3: missing whitespace above this line (too many statements above if) (wsl_v5)
  		ref := s[idx+1:]
  		^
  1 issues:
  * wsl_v5: 1

[Copilot]

@copilot fix this minor lint error

...

Fixed in c525cdb by adding a blank line between the local var declarations and the inner if in parseLibraryReference. Verified locally with golangci-lint run ./cmd/alzlibtool/command/cache/... — 0 issues.