build: prototype source patches for stdlib compat

[cpunion]

Summary

• add a load-time source patch mechanism as a third stdlib compatibility path alongside existing alt patches
• keep the rollout intentionally narrow: only iter is routed through source patch in this PR
• preserve source-patch diagnostics, honor build tags and GOVERSION, and resolve the target GOROOT from go env

What This PR Includes

• source patch package registration in runtime/build.go
• load-time source patch assembly in internal/build
• declaration-level patch behavior:
  • patch files are active by default for source-patched packages
  • //llgo:skipall
  • //llgo:skip ...
  • same-name declarations in patch files override original stdlib declarations
• diagnostics handling:
  • injected patch files carry //line mappings back to the original patch files
  • filtered stdlib files preserve line structure as much as possible
• source patch environment handling:
  • use go env GOROOT instead of the builder binary's compiled-in runtime.GOROOT()
  • skip source patching when the target stdlib package does not exist or is unreadable in the selected toolchain
• tests covering routing, directives, diagnostics, missing-package behavior, and build-tag selection

Current Scope

• iter is the only package switched from alt patch to source patch in this PR
• sync/atomic, unique, internal/reflectlite, internal/runtime/maps, and internal/runtime/sys are kept on their existing non-source-patch paths
• this PR is mechanism-first; broader package migration should be incremental and validated package by package

Proposal

• design proposal: Proposal: add source patch as a third stdlib compatibility mechanism #1760

Validation

• go test ./internal/build -run '^(TestBuildSourcePatchOverlayForIter|TestIterUsesSourcePatchInsteadOfAltPkg|TestSyncAtomicRemainsAltPkg|TestInternalRuntimeMapsRemainsAltPkg|TestInternalRuntimeSysRemainsAltPkg|TestApplySourcePatchForPkg_Directives|TestApplySourcePatchForPkg_MissingStdlibPkg|TestApplySourcePatchForPkg_BuildTaggedPatch|TestApplySourcePatchForPkg_UnreadableStdlibPkg)$' -count=1
• go test ./cl -run 'TestFromTestlibgo/atomic$|TestRunFromTestlibgo/atomic$' -count=1
• LLGO_BUILD_CACHE=0 go run ./cmd/llgo test ./test/std/iter ./test/std/slices ./test/std/maps ./test/std/bytes ./test/std/strings
• LLGO_BUILD_CACHE=0 go run ./cmd/llgo test ./test/std/sync/atomic

[gemini-code-assist]

Code Review

This pull request introduces a source patching mechanism for the LLGo compiler, allowing for the modification of standard library and other packages during the build process via an overlay. It includes logic to parse specific directives like //llgo:skipall and //llgo:nopatch to manage code overrides and stubs. Feedback includes a suggestion to improve the robustness of receiver type parsing by handling unexpected types with a panic and a recommendation to use more idiomatic Go for a package existence check.

[xgopilot]

Well-structured PR introducing a clean source-patch mechanism as a more maintainable alternative to alt-package replacements. The directive system (nopatch/skipall/skip) is elegant, and tests are thorough. A few code-quality issues below — one likely bug in sanitizeSourcePatchDirectiveLines, a duplicate-span logic error, and some dead code to clean up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.44%. Comparing base (04389a9) to head (92a2dc9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1758      +/-   ##
==========================================
- Coverage   88.44%   88.44%   -0.01%     
==========================================
  Files          50       50              
  Lines       13656    13672      +16     
==========================================
+ Hits        12078    12092      +14     
- Misses       1369     1370       +1     
- Partials      209      210       +1     

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