[SizzleUnrlsd]

Closes #185.

Summary

• A template extending another passed every block it defined up the chain as replaced, so the parent's filter applications in an overridden block dropped out of the render context flow even when the child rendered {{ block.super }}, which Django executes. The reachable finding at the parent's line was kept; the exploitable flow was lost.
• _read_scopes now records the blocks whose body renders block.super (_Template.extended), and _reaching passes up as replaced only the blocks a template defines without it (_Template.replaced), level by level: a child with block.super keeps its parent's content; a parent that itself replaced its own parent's block still cuts the chain above it; a replacement at the end cuts the whole chain. Other overridden blocks stay excluded, and each application is yielded once, with the parent template as through and the filter's line as sink_line.
• block.super counts however the block renders it: filtered, conditional or twice.
• Documented in the usage guide and the changelog.

Verification

• RED: 8 failures for the expected reason (the parent flow missing, or the child's own flow alone).
• tests/test_block_super.py: 10 passed; the context-flow, filter-call and docs tests pass. Full suite in the worktree (regression excluded): all passed. ruff check and mypy pass.
• Mutations, all killed: passing every block up as replaced; never recording block.super; treating nothing as replaced (fails the existing overridden-twice case).