Slim AsertoError: single wrapped error, fixed grpc/http correlation, drop dead methods

[gertd]

Summary

• Fixes a live bug: Copy() shallow-copied the errs []error slice, letting siblings derived from the same parent share a backing array and silently corrupt each other's wrapped error once append had slack. Replaced errs []error with a single wrapped error field, which removes the aliasing by construction. azm already hits this pattern via nested Err() calls, so this isn't theoretical.
• Fixes broken grpc↔HTTP status correlation: GRPCStatus() never wrote HTTPCode into the errdetails.ErrorInfo metadata that CustomErrorHandler reads back, so the one thing this package exists to provide only worked because topaz's gerr_middleware.go hand-rolled it a second time. Added ErrorInfo() and fixed GRPCStatus() to be the single source of truth.
• Fixes singleton mutation: FromGRPCStatus no longer mutates the registered singleton *AsertoError in place — it copies first.
• Removes dead code: Int/Int32/Int64/Bool/Duration/Time/FromReader/Interface/SameAs/Equals/WithGRPCStatus/WithHTTPStatus — zero real call sites across go-directory, topaz, and azm (verified by cloning and grepping all three). WithGRPCStatus's one usage in topaz (topazd/authorizer/impl/identity.go) is better served by a dedicated sentinel error than an override.
• Renames StatusCode → GRPCCode for clarity.

This branch was rebased on top of the inner-error/formatting fixes already on main (Fields() recursion into inner errors, .Msg()-before-cause ordering, errors.As-based traversal) — it builds on those, doesn't touch them.

Full design rationale and downstream sequencing notes are in DESIGN.md.

Breaking changes for downstream consumers

topaz and azm will need updates wherever they:

• read .StatusCode (now .GRPCCode)
• call .WithGRPCStatus, .WithHTTPStatus, .SameAs, or cerr.Equals
• chain .Err() more than once expecting accumulation (each .Err() now replaces rather than appends — only the test suite here exercised that, no downstream repo does)

Test plan

• go build ./...
• go vet ./...
• gofmt -l . clean
• go test ./... -race passes
• Manually reproduced and verified the fix for the Copy() aliasing bug (siblings derived from a 3-deep error chain now stay independent)
• Manually reproduced and verified the fix for the FromGRPCStatus singleton mutation
• Verified Fields() still recurses into inner AsertoErrors under the new single-wrapped-error model

🤖 Generated with Claude Code

[coveralls]

Coverage Report for CI Build 32731463535

Coverage increased (+4.1%) to 68.465%

Details

• Coverage increased (+4.1%) from the base build.
• Patch coverage: 6 uncovered changes across 1 file (46 of 52 lines covered, 88.46%).
• 19 coverage regressions across 1 file.

Uncovered Changes

File	Changed	Covered	%
errors.go	52	46	88.46%

Coverage Regressions

19 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
errors.go	19	78.87%

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Coverage Stats

Relevant Lines:	241
Covered Lines:	165
Line Coverage:	68.46%
Coverage Strength:	0.76 hits per line

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💛 - Coveralls