fix: only rewrite whole Windows reserved device names

[Pranav-IIITM]

Summary

paths.FilterReservedNames rewrote valid names because it matched the Windows reserved words (aux, com, con, lpt, nul, prn) as substrings, and it ran on every OS. As a result, ordinary names were mangled — including on Linux and macOS, where no names are reserved — which changed the Docker cache volume names users actually asked for.

Before / After

                     before                after
my-company-cache  -> my-c_o_mpany-cache -> my-company-cache
connection-cache  -> c_o_nnection-cache -> connection-cache
config            -> c_o_nfig           -> config
nul   (Windows)   -> n_u_l              -> n_u_l
nul.txt (Windows) -> n_u_l.txt          -> n_u_l.txt
nul   (Linux/mac) -> n_u_l              -> nul   (unchanged)

What changed

• Skip the transformation entirely on non-Windows platforms — these names are only reserved on Windows.
• Match the whole base name (the part before the first .), case-insensitively, instead of matching a reserved word anywhere in the string.
• Use the accurate Windows reserved set: CON, PRN, AUX, NUL, COM1–COM9, LPT1–LPT9. Bare com/lpt are no longer rewritten, since they are not actually reserved.
• Preserve original casing and any extension, so nul.txt becomes n_u_l.txt.
• Replace the old test that asserted the substring behavior with cases covering Windows reserved names, Windows ordinary names, and the non-Windows no-op.

Impact

• Cache volume names now match what the user requested, so they are recognizable in docker volume ls and via scripting.
• A user-pre-created volume whose name merely contains a reserved word (e.g. my-company-cache) is no longer missed.
• Ordinary names are no longer needlessly rewritten on Linux and macOS.

Migration note

Existing incorrectly mangled volumes are not renamed automatically. After upgrading, pack may reference a new, correctly named volume while the old mangled volume remains until it is cleaned up manually.

Testing

• go build ./internal/paths/...
• go vet ./internal/paths/
• go test ./internal/paths/... ./pkg/cache/...

Closes #2703