Lambe 0.6.1: completer trailing-whitespace fix; rumil 0.6.0 dep bump

[hakimjonas]

Fixes the tab-completion corruption where trailing whitespace in the REPL (or the arda-web playground) produced a wrong replacement offset. Typing .dependencies, a space, then Tab now completes against .dependencies instead of producing ..dependencies.

Completer changes (lib/src/completer.dart):

• Unparsed-remainder classification no longer uses regex. Two small Rumil parsers (_pipeCtx, _fieldTailCtx) handle pipe-op and field-tail contexts, with position() for offset tracking.
• AST-end offset (astEnd) is computed by walking back over trailing whitespace in the successfully-parsed prefix. Previously before.length was used directly, which overshoots when the user has typed trailing whitespace.
• Whitespace handling is now uniform across space, tab, CR, LF.

Tests:

• New 14-test Trailing whitespace regression group in test/completer_test.dart covering identity tails, field tails, access tails, parameterized ops, pipe-op paths, and the no-trailing-whitespace parity cases.
• Fixed two cursor positions in existing tests (21 and 19 instead of 22 and 20) to match the new contract.
• 752 tests passing, no regressions.

Investigation artefacts:

• PLAN_COMPLETER_WHITESPACE_FIX.md documents the characterization (9 scenarios, 4 hypothesized fix shapes) and the decision to use Rumil parsers over regex.
• tool/probe_completer.dart is a diagnostic runner for the cases the plan enumerates.

Dependency bumps (pubspec.yaml):

• rumil: ^0.5.0 -> ^0.6.0
• rumil_parsers: ^0.5.0 -> ^0.6.0
• rumil_expressions: ^0.5.0 -> ^0.6.0

Rumil 0.6.0 adds the position() primitive used by the completer fix.

Version metadata:

• pubspec.yaml version 0.6.0 -> 0.6.1.
• lib/src/_version.dart regenerated via dart run tool/gen_version.dart.
• doc/lam.1.md front-matter updated, doc/lam.1 regenerated.
• README.md and doc/getting-started.md: stale 0.5.0/0.6.0 version strings in example outputs updated.

Release gate: format, analyze --fatal-infos, dartdoc --dry-run, tests all clean.