[vector_graphics][vector_graphics_compiler] Fix text-anchor and tspan gap with adjacent <tspan>s (#11637)

## Summary

`<text x="..." text-anchor="middle"><tspan>ABCDEFG</tspan><tspan>ABCDEFG</tspan></text>` rendered noticeably differently from the equivalent single-tspan `<tspan>ABCDEFGABCDEFG</tspan>`: the two-tspan version was shifted right past the anchor *and* had a visible space between the two tspans. Two independent root causes, fixed in two separate places.

**1. `vector_graphics` — anchor applies per-chunk, not per-tspan.** The listener was independently offsetting each tspan's paragraph by `dx - paragraphWidth * xAnchorMultiplier`. Per the SVG spec, `text-anchor` applies to the entire anchored chunk (the contiguous run of glyphs whose start is established by an explicit `x`/`y`), not to each tspan in isolation. The listener now buffers per-tspan paragraphs within a chunk, flushes on the next explicit position update or on `toPicture()`, and applies the anchor offset to the chunk total. `text-anchor="start"` behavior is unchanged. A new TextPosition with `x` (or `reset=true`) starts a new chunk; bare per-tspan TextPositions emitted by the parser do not.

**2. `vector_graphics_compiler` — parser injected a spurious space between adjacent tspans.** `_appendText` had a "previous element was a tspan ⇒ prepend a space" rule that fired regardless of whether the source actually had whitespace at the boundary. So `<tspan>A</tspan><tspan>B</tspan>` parsed to `['A', ' B']` (the visible gap), even though every browser parses it to `'AB'`. Replaced with a rule that only prepends when whitespace truly exists at the boundary — either as a leading-whitespace prefix on the current text, or as an earlier whitespace-only text event we've now flagged.

Tests added on both sides:
- `vector_graphics/test/listener_test.dart`: `Text anchor middle centers the entire chunk across tspans` — drives the listener with a two-tspan middle-anchored chunk (including the parser's bare per-tspan TextPosition between them) and asserts the second tspan starts at the original x.
- `vector_graphics_compiler/test/parser_test.dart`: `adjacent tspans without whitespace are not separated by a space` and `adjacent tspans with whitespace between still get a space` — locks down both branches of the new parser rule.

Both packages bumped to `1.2.1` with CHANGELOG entries.

Fixes flutter/flutter#185927

## Before / After

| SVG | Before (buggy) | After (this PR) |
| --- | --- | --- |
| **svg1** (single tspan)<br>`<svg xmlns="http://www.w3.org/2000/svg" width="200" height="100" fill="none">`<br>&nbsp;&nbsp;`<text x="100" y="50" text-anchor="middle" fill="#FF0000">`<br>&nbsp;&nbsp;&nbsp;&nbsp;`<tspan>ABCDEFGABCDEFG</tspan>`<br>&nbsp;&nbsp;`</text>`<br>`</svg>`<br><br>**svg2** (two tspans)<br>`<svg xmlns="http://www.w3.org/2000/svg" width="200" height="100" fill="none">`<br>&nbsp;&nbsp;`<text x="100" y="50" text-anchor="middle" fill="#FF0000">`<br>&nbsp;&nbsp;&nbsp;&nbsp;`<tspan>ABCDEFG</tspan><tspan>ABCDEFG</tspan>`<br>&nbsp;&nbsp;`</text>`<br>`</svg>` | <img width="390" height="844" alt="image" src="https://github.com/user-attachments/assets/7ffbe594-d821-4b4c-b759-c5fb61a61339" /> | <img width="780" height="1688" alt="image" src="https://github.com/user-attachments/assets/076b69fb-2bfc-490a-9669-566e45f7e9a2" /> |

## Test plan

🤖 Generated with [Claude Code](https://claude.com/claude-code)