test(formatted): add regression tests for quoting scalar values

[stevebeattie]

Summary

This adds TestQuoteScalars, a table-driven regression test for how quote
expressions (--quote / quote: in .yam.yaml) handle each kind of YAML
scalar. Only tests change; formatter behavior stays the same.

yaml.v3 already does the right thing here. When a scalar on a quote path isn't
explicitly tagged, it drops the implicit tag and writes the value
double-quoted with its original text, so 5.12, 42, 0x1F, .inf, true
and 2024-01-01 all become strings with no type coercion in yam. Until now
only a single float fixture (testdata/format/quotes.yaml) covered quoting,
and it didn't check other types, explicit tags, or the decoded type. That's
how #125 came to propose a fix that wasn't needed.

What's covered

Each row checks three things: the exact formatted text, the decoded value
(so a change that keeps the text but changes the type fails), and that
formatting the output a second time changes nothing.

• Implicit scalars become quoted strings with their text unchanged:
  float, int, hex and octal ints, .inf, .NaN, exponent floats, 1_000,
  true, timestamps, and block literals.
• Strings: plain strings, YAML 1.1 bool words (yes), single-quoted
  strings, and already double-quoted strings (left unchanged).
• Explicit tags are kept: !!float 5.12 becomes !!float "5.12", and
  the same for !!int, !!bool and !!str. Quoting should never override
  a type the author wrote out.
• Things that stay attached: anchors and line comments on quoted values.
  Nulls stay null.
• Paths: several quote expressions all apply; .v[] quotes every
  sequence item; .v[1] quotes only that item.
• Paths that don't match leave values unchanged: a sibling key, a
  same-depth path under a different parent (the mistake in the yam --quotes doesn't work for floats (and probably others) #100 repro),
  a deeper path, and a path to a sequence rather than its items.

The block-literal row deliberately pins today's behavior: quoting a |
scalar flattens it to one escaped line. A comment on the row says so.

Verification

• go test ./... and go test -race ./pkg/yam/formatted pass on main.
• With quotes: Fix quotes on ints and floats #125's encoder change applied, exactly two rows fail
  (!!float "5.12" → !!str "5.12", !!int "42" → !!str "42").
• I broke the code on purpose in several other ways, and each one fails
  the row written for it. Before this test, the first five passed the
  entire repo suite:
  • path matching that compares only the last key
  • applying only the first or only the last quote expression
  • quoting nulls into "null"
  • dropping line comments from quoted values
  • losing explicit tags (|= changed to =)
  • skipping quoting for strings
• gofmt -l . is clean. golangci-lint run --new-from-rev=origin/main
  reports 0 issues.

Related: #100, #125

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