feat: implement slog.LogValuer on stacktrace error type (#16)

[sridhar-3009]

Problem

Stacktrace errors currently appear as a flat, multi-line string in log/slog output. Since Go 1.21 introduced log/slog and the slog.LogValuer interface, types can control how they are represented in structured logs. Implementing LogValue() on the *stacktrace type allows callers to get a properly structured log entry instead of a raw Error() dump.

Closes #16.

Changes

slog.go — new file

Implements slog.LogValuer on *stacktrace:

func (st *stacktrace) LogValue() slog.Value {
    return slog.GroupValue(
        slog.String("message", st.Error()),
        slog.Any("frames", collectFrames(st)),
    )
}

The LogValue() method returns a slog.GroupValue with two keys:

Key	Value
message	Full human-readable error string (same as err.Error())
frames	Slice of per-frame groups, each with function, file:line, and the frame-local message when present

slog_test.go — new file

Three new tests:

• TestLogValue — asserts the interface is implemented and the group contains message and frames keys
• TestLogValuePropagated — asserts propagated errors carry both outer and inner messages
• TestLogValueInSlogHandler — logs through a slog.JSONHandler and asserts structured keys appear in the JSON output

go.mod / go.sum — new files

The repository previously had no go.mod. Adding one is required to import log/slog (Go 1.21+) and to enable module-aware builds. The module path matches the existing import path used throughout the code: github.com/palantir/stacktrace.

Example structured output

With a JSON handler:

{
  "level": "ERROR",
  "msg": "operation failed",
  "error": {
    "message": "outer context\n --- at pkg/foo.go:42 (Fn) ---\nCaused by: inner error\n --- at pkg/bar.go:10 (Bar) ---",
    "frames": [
      {"message": "outer context", "function": "Fn", "file": "pkg/foo.go:42"},
      {"message": "inner error",   "function": "Bar", "file": "pkg/bar.go:10"}
    ]
  }
}

Test plan

• go test -run TestLogValue ./... passes (3 new tests)
• All other previously-passing tests still pass
• go build ./... succeeds with no warnings