fix(genspec-tui): drop the YAML position workarounds, upgrade core to v0.0.3

[fredbi]

The spec pane lays the lexer's tokens out by position to colour them. Two workarounds stood between it and the YAML lexer, both now fixed upstream in go-openapi/core v0.0.3.

A YAML block collection has no "{" / "[" / "}" / "]" character, so the lexer had nothing to point its container delimiters at. It reported the opener at the first entry's separator — a line BELOW the key it precedes — and the closer at line 0 column 0, which is not a position at all. The accumulator therefore had to sort each line's runs by column and drop the positionless ones. Both are gone: delimiters now take the span of what they enclose, so the stream is emitted in non-decreasing position order and every token carries a real position.

Non-decreasing, not strictly increasing. A block delimiter has no characters of its own, so it shares its column with the token that does own the text there: the opener with the first token inside, the closer with the last. Appended naively that makes a zero-width run followed by one painting its neighbour as punctuation — - b and ok: true would render their value as a delimiter. addSpan now collapses same-column runs, and the token with characters wins. Neither first-wins nor last-wins would do, because the opener precedes its neighbour while the closer follows it.

Verified over the 283 golden specs in both renders, ~97k tokens: no positionless token, no backwards step, and every same-position pair involves a delimiter — two content tokens never collide. The pointer and $ref indexes built from those documents are byte-identical before and after the upgrade, so only the spans moved.

Visible effect: YAML separator colouring becomes uniform. It had been accidental — the borrowed positions punctuation-coloured the ":" of a mapping's first entry and the "-" of a sequence's first item, and nothing else. The YAML lexer elides separator tokens by design, so ":" and "-" now take the preceding run's class everywhere. The spec pane's JSON render is unaffected: its lexer emits real delimiter tokens at real columns and never collided.

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Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

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• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.