feat(version): port rpm-style version comparison from ZETA

.omo/notepads/tofu-core/learnings.md

@@ -6,6 +6,46 @@ _Auto-scaffolded by /start-work. Append new entries below - never overwrite._
 
 ---
 
+## Task 5 — tofu.vercmp (RPM-style version comparison)
+
+### Module naming
+- `version` is a D reserved keyword (conditional compilation). Module is named
+  `tofu.vercmp`, file `src/tofu/vercmp.d`.  The `DepConstraint` struct in
+  `types.d` uses field `ver` (not `version`) for the same reason.
+
+### Algorithm (exact port from ZETA/lib/vercmp.lua)
+- `nextSegment`: skips any non-digit/non-letter chars (separators); then reads
+  a contiguous run of same-type chars.  Returns `(segment, nextIndex)`.
+- `cmpNumeric`: strips leading zeros from both segments (`"007"` → `"7"`,
+  `"000"` → `"0"`).  Longer stripped string wins; then lexicographic.
+- `compare`: strips all whitespace, then loops over alternating segments.
+  If both segments are digit-typed → `cmpNumeric`, else lexical.  When one
+  version exhausts segments: the exhausted one is *older* (shorter = older).
+  This matches RPM semantics: `compare("1.0.0", "1.0") > 0`.
+- `satisfies`: uses `final switch` on `DepOp` enum.  Maps to `compare` result
+  as expected (ge→c>=0, le→c<=0, eq→c==0, ne→c!=0, gt→c>0, lt→c<0).
+- `parseDep`: delegates to `DepConstraint.parse` (already ported in types.d by
+  task 4).  Both implementations are kept consistent.
+
+### whitespace stripping
+- Could not use `filter`+`array` because Phobos auto-decodes `string` to
+  `dchar` range elements, returning `dchar[]` not `string`.  Implemented a
+  manual `removeWhitespace` helper using raw code-unit indexing, with a
+  `@trusted` cast since the freshly-allocated `char[]` has no aliasing.
+
+### Test porting
+- All 10 ZETA test categories ported as separate `@safe unittest` blocks.
+- 4 modules pass unittests (types, vercmp, plus pre-existing ones).
+- `dub build` passes with warnings-as-errors enabled.
+
+### Future considerations
+- When task 12 (constraint resolution) imports `tofu.vercmp`, the `compare`
+  and `satisfies` functions are ready.  The `parseDep` integration with
+  `DepConstraint.parse` ensures consistency between ZETA and tofu's dep
+  constraint semantics.
+
+---
+
 ## Task 2: Config Loading (src/tofu/config.d)
 
 ### toml package API (v1.0.0, Kripth/toml)