[beffe123]

Before submitting your PR, please confirm the following.

• Describe the purpose for which you created this PR.
• Create test code that corresponds to the modification

Purpose

MultiLineState.validateIndentColumn is called for every character of a literal or folded block scalar, and it called
firstLineIndentColumnByOpt(s.opt) each time. For a header without an indentation indicator (|, >, |-, >+, …),
that is strconv.ParseInt(""), which returns a *strconv.NumError: one allocation of 48 bytes per character of the text.

This PR parses the indicator once, in setLiteral and setFolded, and keeps it in MultiLineState.optIndentColumn.
Behaviour does not change.

Unmarshalling a document with a literal block of 2,030 characters into map[string]any (Apple M2, Go 1.26.9, 6 runs):

	before	after
time	98.8 µs	53.9 µs
memory	123.9 KB	26.3 KB
allocations	2,186	155

We came across it while profiling a CMS that parses a few thousand pages with text blocks on every publish: two thirds of
all bytes allocated were these errors.

Test

TestBlockScalarAllocations (lexer) tokenizes block scalars of 10 and of 10,000 characters and requires about the same
number of allocations. Before the change it was 67 against 20,054, after it 46 against 52. make test (with the YAML test
suite) and golangci-lint run pass.

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.66%. Comparing base (edee2f9) to head (9bebe2b).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #986   +/-   ##
=======================================
  Coverage   80.65%   80.66%           
=======================================
  Files          22       22           
  Lines        6845     6847    +2     
=======================================
+ Hits         5521     5523    +2     
  Misses        905      905           
  Partials      419      419           

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