Add Path.relative for lexical relative-path computation

[carpentry-agent]

Adds Path.relative, the lexical companion to absolute: it computes the path to a target relative to a base without touching the filesystem — the staple operation behind Python's os.path.relpath, Rust's pathdiff, and Java's Path.relativize.

Behaviour

(relative target base) returns a (Maybe String):

call	result
(relative "/a/b/c" "/a/b")	Just "c"
(relative "/a/b" "/a/b/c")	Just ".."
(relative "/a/x" "/a/b")	Just "../x"
(relative "/a/b" "/a/b")	Just "."
(relative "/a/b/c" "/x/y")	Just "../../a/b/c"
(relative "/a/b" "a/b")	Nothing (absolute vs relative)

It composes the existing primitives: both inputs run through normalize (so ./../redundant separators are resolved first), are split into components, stripped of the empty-string artifacts split yields at roots, then reduced to a common leading prefix plus one .. per leftover base component followed by the target's remainder. Nothing is returned when the paths cannot be related lexically — one absolute and one relative, or (on Windows) different drive roots.

Naming

The natural name would be relative-to, but core's Dynamic.carp already defines a defndynamic relative-to (the load-path helper behind add-c/add-cflags). That dynamic binding shadows the name for unqualified calls, so (relative-to …) after (use Path) expands the macro instead of calling the function and fails to type-check. Since the module is built for unqualified use (use-all Path throughout the tests), I named it relative, which also pairs cleanly with the existing absolute. Happy to switch to a qualified-only relative-to, or another name, if you'd rather.

Tests / CI

test/path.carp gains nine assert-equal cases covering every example above plus the relative→relative and reverse-mismatch cases. Local run: 56 passed, 0 failed. carp-fmt --check and angler are clean; committed docs regenerated via gendocs.carp.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[hellerve]

verified locally: 47/0 -> 56/0, and every row of the behaviour table reproduces exactly, including Nothing for absolute-vs-relative. the naming constraint is real (core Dynamic.carp:119 defines defndynamic relative-to); relative it is, pairing with absolute.