Grammar and AST support for the ^ reverse / parent navigation key

[jtnelson]

Tracks the CCL side of reverse / parent navigation. Parent feature:
cinchapi/concourse#781.

Summary

Add a reverse-navigation marker ^ to navigation keys so a stop can be
traversed backward, to the parent: the records that link to the current
record. ^ alone means the parent via any key; ^key means the parent via
that key. Reverse stops compose with forward stops, the transitive * suffix,
per-stop bracket timestamps, and scope prefixes.

Scope

• Lexer: accept a leading ^ on navigation-key segments in the key-bearing
  tokens (PERIOD_SEPARATED_STRING, ASTERISK_SUFFIXED_STRING,
  NAVIGATION_SCOPE_OPEN) and a standalone ^ segment. Disambiguate a ^-led
  key from a ^-led unquoted value.
• AST: add a reverse direction to NavigationKeyStop (alongside the transitive
  flag and timestamp), with the key optional for bare ^; propagate through
  NavigationKeySymbol (stop parsing, components, base key, parameter
  stripping, scope-prefix parsing).
• Regenerate the parser from the grammar; do not hand-edit generated sources.
• Update CCL_REFERENCE.md (Navigation Keys section).

Behavior / contract

• ^key round-trips through parse and toString; ^ is a valid single-segment
  navigation key.
• Composes: ^.a, ^key.a, ^*, ^[t], ^.(...).
• The reverse marker names the source key, or none for bare ^. There is no
  auto-paired inverse.

Acceptance Criteria

• Grammar accepts ^, ^key, and reverse stops mixed with forward stops,
  *, [t], and scope prefixes.
• NavigationKeyStop and NavigationKeySymbol model the reverse direction
  and round-trip.
• An unquoted value beginning with ^ still parses, or is documented to
  require quoting.
• Tests and CCL_REFERENCE.md updated.