fix: ^0, ^0.0 and ~1 read unwritten version segments as zeros and bound the range far too tightly #864

Reproduction

A dependency constraint whose version leaves segments unwritten is bounded as if the missing segments were zeros:

[dependencies]
foo = "^0"     # expected: any 0.x.y
bar = "^0.0"   # expected: any 0.0.y
baz = "~1"     # expected: any 1.x.y

mcpp::version_req::matches (modules/versioning/src/version_req.cppm) gives:

With a project [indices] entry publishing acme.gadget 0.3.0 and 0.4.0 (the e2e 169 layout), gadget = "^0" under [dependencies.acme] gives:

$ mcpp build
error: dependency 'acme.gadget': constraint '^0' matches none of: [0.3.0, 0.4.0]

gadget = "~0" fails the same way. ~1 against 1.0.0 and 1.1.0 selects 1.0.0, because 1.1.0 is outside its bound.

The caret branch picks the segment to bump from major(), minor() and patch(), which read an unwritten segment as 0, so ^0 falls through to bumping the patch. The tilde branch always bumps the minor, so ~1 becomes <1.1.

Expected

The grammar is documented as Cargo's (docs/05-dependencies.md: ^1.2.3 is caret, ~1.2.3 is tilde), and mcpp.xpkg_version already reads a partial caret this way (^0 gives <1). A segment that is not written is a wildcard:

• ^0 → >=0.0.0, <1.0.0; ^0.0 → >=0.0.0, <0.1.0; ^0.0.0 stays <0.0.1
• ~1 → >=1.0.0, <2.0.0; ~1.2 and ~1.2.3 stay <1.3.0

Actual

The bounds in the table above.

Environment

• mcpp version: 2026.10.29.1 (main at db3b815)
• OS: any; the logic is host-independent