fix: allow keywords as block type names and labels

[aoskotsky-amplify]

Problem

Keywords parse as attribute names but not as block names:

in = 1        # works today

in { }        # UnexpectedToken
for_each { }  # UnexpectedToken
resource in { }  # UnexpectedToken

Keywords (in, for, if, for_each, else, endif, endfor) and literal keywords (true, false, null) are reserved only inside expressions. In a body they are ordinary names, valid as a block type or label just as they already are as attribute names.

The grammar already encodes this for attributes and simply never carried it to blocks:

attribute : _attribute_name EQ expression
_attribute_name : identifier | keyword | literal_value   ← keywords allowed
block : identifier (identifier | string)* ...            ← but not here

This continues the line of #164 (attributes named in) and #168 (if/for_each as identifiers), which fixed the attribute side only.

Fix

One line of grammar — reuse the existing _attribute_name rule in the block type and label positions:

block : _attribute_name (_attribute_name | string)* new_line_or_comment? LBRACE body RBRACE

No new rule, so nothing to keep in sync. Because the rule is inlined, block() now receives KeywordRule/LiteralValueRule for bare names, so the transformer normalizes them to IdentifierRule — exactly what attribute() already does — keeping BlockRule.labels a single type for downstream consumers.

Testing

• New resource_keyword_block integration suite (the block-side sibling of the existing resource_keyword_attribute), covering keyword block types, a keyword label (resource in), nesting, and a string label. All four pipeline files included.
• Verified the fixture genuinely guards the fix: reverting the source with the fixture present produces 11 failures.
• Full suite green: 1518 tests, 0 failures, unchanged from baseline.
• Expression keywords confirmed still reserved: for-tuple, for-object, conditional, for_each =, and template %{ for } / %{ if } directives all parse and reconstruct as before.
• pre-commit clean (ruff, ruff format, mypy).

Note for reviewers

Sharing _attribute_name also permits true { } / null { } as block names. That matches HCL, where a block type is an Identifier and these are contextual keywords, and it matches how they are already accepted as attribute names. Easy to restrict to identifier | keyword at the block position if you would rather not accept it.

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