feat(parser): support ES6-style shorthand entries in config/schema literals (fixes #1275)

[Peefy]

Summary

Adds ES6-style shorthand for config/schema literals (issue #1275). A bare
identifier inside { ... } now stands for key = key:

schema Person:
    name: str
    age: int

name = "next"
age  = 30

p = Person { name, age }          # === Person { name = name, age = age }
m = { name, age }                 # === { name = name, age = age }
n = Person { name, age = 99 }     # mixed shorthand + override

This is purely additive: every existing entry carries an explicit
separator, so any program that parses today still parses tomorrow. The
new arm fills the parser-error gap that appeared when the separator was
missing.

Implementation

• AST (crates/ast/src/ast.rs): new is_shorthand: bool field on
  ConfigEntry with #[serde(default, skip_serializing_if = "is_false")]
  so existing AST-JSON fixtures continue to deserialize.
• Parser (crates/parser/src/parser/expr.rs): parse_config_entry
  now reuses the parsed identifier as both key and value when no
  separator follows a single-segment Identifier key. Dotted keys
  like { a.b } still hit the existing "expected one of :/=/+="
  diagnostic so ConfigNestAttrTransformer key-flattening keeps its
  current semantics.
• Formatter (crates/ast_pretty/src/node.rs): write_entry short-
  circuits on is_shorthand to round-trip {name} exactly. We never
  rewrite explicit name = name into shorthand (avoids silently
  flipping semantics inside schema bodies where name could resolve
  to a local).
• Struct-literal call sites: is_shorthand: false added in
  crates/parser/src/parser/expr.rs (3 sites in parse_body_item),
  crates/sema/src/pre_process/config.rs,
  crates/query/src/override.rs (3), and
  crates/tools/src/vet/expr_builder.rs (4).

No evaluator, walker, sema transformer, or loader changes needed —
existing consumers walk key + value and ignore the new flag.

Tests

• 4 new parser snapshot tests (config_shorthand_stmt_0..3) covering
  the all-shorthand, mixed, and explicit-baseline cases.
• Existing config_recovery_0..15 snapshots regenerated to include the
  new field (no behaviour change; just the additive is_shorthand: false).
• 3 new grammar tests under tests/grammar/schema/:
  • shorthand/ — positive case
  • shorthand_dotted_fail/ — dotted-key rejection
  • shorthand_format/ — formatter round-trip
• cargo test -p kcl-parser → 514 passed, 0 failed
• cargo test -p kcl-tools --lib → 60 passed, 0 failed
• cargo test -p kcl-query --lib → 15 passed, 0 failed
• cargo test -p kcl-sema --lib → 61 passed, 0 failed

Out of scope (follow-ups)

• Positional / type-based shorthand (DiskOptions { 32, "ssd" }).
• Shorthand for dotted keys ({a.b}).
• Canonicalizing explicit name = name → {name} via kcl fmt.

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