session reversion: rewrite all version operators unconditionally (match bash)

[dkijania]

Fixes #12.

Problem

deb-session-reversion.sh — which mina's callers (reversion.sh, manager.sh) invoke with no flag — rewrites the old version to the new one across every versioned dependency constraint on every reversion. deb-toolkit session reversion did this only under --update-deps, and even then only for exact = pins, leaving >=/<</etc. alone on the theory that a looser constraint is still satisfied by a higher version.

That theory breaks for mina: reversions are not monotonic. manager.sh reversions from $__source_version to an arbitrary $__target_version — e.g. a fork build 3.4.0-rc2-company-<hash> down to a clean 3.4.0-rc2. On a downward reversion a left-alone (>= old) is unsatisfiable and the package won't install.

Changes

• replace_in_constraint rewrites all Debian relation operators (=, <<, <=, >=, >>), matching the operator as a prefix rather than splitting on whitespace — so both (<< X) (spaced) and (=X) (no space) are handled. mina emits both forms.
• The dependency rewrite is now unconditional. The --update-deps CLI flag and the update_deps manifest field are kept, accepted, and ignored (deprecated), so existing callers and manifests keep parsing/working; the flag emits a deprecation warning. bash has no "version-only" reversion mode, so matching bash means always rewriting.
• The rewrite stays constraint-scoped (only inside (...)), so a version-like substring in a package name is never mangled — an improvement over the bash bare-substring pass, and identical output on mina's real (parenthesized-constraint) inputs.

Verification

Against the bash scripts, realistic downward reversion 3.4.0-rc2-company-abc123 → 3.4.0-rc2, mixed (=X)/(<< X), no flag:

              bash                              deb-toolkit
Version:      3.4.0-rc2                         3.4.0-rc2
Depends:   mina-devnet-config (= 3.4.0-rc2)     mina-devnet-config (= 3.4.0-rc2)
Replaces:  mina-mainnet (<< 3.4.0-rc2)          mina-mainnet (<< 3.4.0-rc2)
Breaks:    mina-mainnet (<< 3.4.0-rc2)          mina-mainnet (<< 3.4.0-rc2)
Conflicts: mina-mainnet (<< 3.4.0-rc2)          mina-mainnet (<< 3.4.0-rc2)

Control fields identical. (Before this change, deb-toolkit left every one of those deps at the old version — even with --update-deps.)

• New unit tests: all-operator rewrite; no-space operator ((=X)).
• Updated session_roundtrip integration test, which previously asserted >= should be left alone — that assertion encoded the bug.
• Full suite green: cargo test --all-targets (52 unit + integration), fmt, clippy -D warnings.

Not in scope

The secondary save divergence noted in #12 — deb-toolkit drops the leading ./ and root entry in data.tar vs dpkg-deb --build — is untouched here; that's a save concern, not reversion. #12 tracks it.