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.

Note: Lambe 0.6.0 was tagged locally but not published to pub.dev.
The pub.dev jump is 0.5.0 -> 0.6.1; consumers will see the full
0.6.0 + 0.6.1 CHANGELOG entries on install.