refactor: drop nixpkgs lib dependency (pure builtins tree reader)

[sini]

Summary

Removes the nixpkgs lib dependency from the tree reader. The library core is now pure builtins — import-tree stays a zero-dependency flake (no new inputs).

The seven lib helpers previously used were handled two ways:

• lib.pipe — restructured away, not vendored. Both pipe chains became plain let-bindings. The makeRelative chain (which built a list of isFileRelative partial applications and fold-applied over it) collapses into a direct builtins.foldl' (acc: root: isFileRelative root acc) over the root dirs — a genuine simplification.
• Genuine primitives vendored inline, each with a provenance comment naming its nixpkgs origin: flatten, hasPrefix, removePrefix, hasSuffix, hasInfix, listFilesRecursive. hasInfix is a literal substring scan (no regex-escaping surface); behavior-identical for its sole internal use.

Every vendored util was cross-checked against nixpkgs.lib over sampled and adversarial inputs (regex-meta chars, newlines, empty/oversized infixes) — no divergences. listFilesRecursive output is byte-identical (order + set).

Behavior change

The withLib / lib-threading machinery existed only to obtain those helpers, so it dissolves (no lib == null throw, no leafs = lib: threading, no module = { lib, ... }: delay trick). As a result, .leafs, .files and .pipeTo now work everywhere with zero setup — no withLib call needed, including outside module evaluation.

withLib is retained as a silent no-op so existing it.withLib pkgs.lib callers keep working; it is removed from the docs. The emitted module stays a lambda, so the tree read remains deferred to module-eval (constructing it ./dir does not walk the filesystem).

Tests & docs

• 34/34 tests pass; treefmt clean; just ci green.
• The old "fails if no lib has been set" test (which asserted the now-removed throw) is replaced with tests that the reader works without withLib and that withLib remains a working no-op.
• All withLib references removed from docs; combinator-guide examples reworked to lib-free demo operations.