feat: emit per-benchmark map.json under data_mapping/

[AlexsanderHamir]

Summary

• Adds .prof/<tag>/data_mapping/<Benchmark>/map.json — a machine-readable index of hotspots, call_trees, source_lines, measurements, and profiles for LLM/agent navigation.
• New internal/datamap package owns the JSON schema and pure builder; collect wiring is a single emit call per benchmark (auto + manual).
• Paths in JSON are relative to the tag root; top symbols and function inventory come from existing ProfileData (no extra pprof runs).

Motivation

Agents currently scan .prof/ or read large hotspot files to discover artifacts. map.json provides semantics, reading order, and triage summaries in one file.

Test plan

• go test ./internal/workspace/ ./internal/datamap/ ./engine/collect/ -count=1
• go test ./...
• golangci-lint run ./...
• prof auto on one benchmark; confirm map.json validates and paths resolve relative to tag root

Out of scope

Tag manifest, triage_hint synthesis, JSON Schema file.

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