fix(core): keep the mixed parser's prose identical to the stream

Fixes #423

createMixedStreamParser called onText without the line break and skipped blank lines, so the prose it rebuilt differed from what createJsonRenderTransform streamed for the same text. Storing the raw transcript and re-splitting it on reload could therefore show different prose than the live stream.

Change

packages/core/src/types.ts

• onText now receives the text exactly as the transform emits it: a prose line arrives with its trailing "\n", a blank line arrives as a single "\n", and the ```spec fence lines are swallowed together with their own newline (they were already swallowed, but without also removing the surrounding blank-line semantics). flush() mirrors the transform's flushBuffer: an unterminated final line keeps its own bytes and gets no added newline.
• New splitSpecText(text): { segments, invalid }, exported from the package: the same split for a complete string, in document order, with adjacent prose merged. invalid reports lines inside a fence that do not parse as patches (the transform drops them).
• New exported types SpecTextSegment, SpecPatchSegment, SpecSegment, SplitSpecTextResult.

Tests

packages/core/src/types.test.ts

• createMixedStreamParser > reproduces the prose the transform streams for a fenced spec: feeds the issue's example through both the parser and createJsonRenderTransform and asserts the concatenated onText output equals the transform's prose (and equals "Here is your report.\n\n\nLet me know.").
• createMixedStreamParser > keeps blank lines as paragraph breaks (was skips empty lines): "\n\n\n" now yields three "\n" callbacks.
• splitSpecText: the same prose as the transform, plus invalid for a non-patch line inside a fence.
• Existing assertions that expected newline-less onText/texts values were updated to the new, transform-matching values ("Hello" -> "Hello\n" etc.).

Verified: pnpm exec vitest run packages/core/src/types.test.ts -> 131 passed; with types.ts reverted, 7 tests fail (the parser tests above and the existing text assertions). tsc -p packages/core/tsconfig.check.json exits 0.