tp: scope pprof archive members and intern their fallback mappings

An archive with dozens of pprofs currently imports as an undifferentiated
blob: every profile lands on the hard-coded "pprof_file" scope. Scope each
profile by its source file instead: TraceFileTracker gains a CurrentFile()
accessor and the pprof reader walks the file-nesting chain past the gzip
layer to the nearest named ancestor. The file id is captured on the first
Parse() call -- ParseProfile runs from OnPushDataToSorter, which may fire
after the end-of-file notifications have popped the trace-file parsing
stack, so reading CurrentFile() there would make scope attribution depend
on pipeline ordering.

Locations without a pprof mapping each allocated a fresh dummy mapping, so
identical frames never interned across profiles and callsites multiplied
by the profile count: a 32 MB / 1000-profile archive peaked at 1.8 GB and
OOMed the Wasm heap. Intern the shared fallback mapping and skip
re-symbolizing interned frames; the same archive now peaks at 647 MB.

Diff tests lock the behaviour in: zip/tar members are scoped by member
file name (walking past the unnamed gzip layer), directly opened pprofs
keep the "pprof_file" scope, duplicate archive members do not duplicate
frames/mappings/symbols, and mapping-less locations share a single
interned "[unknown]" fallback mapping (asserted with a minimal
hand-encoded pprof, since no checked-in pprof lacks mappings).