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fix: share one scratch buffer in diag_section_dispatch to stop httpd stack overflow (#1154)

diag_section_dispatch() (platform/espidf/bb_diag_http/bb_diag_http_section_dispatch.c)
declared two BB_DIAG_SECTION_SCRATCH_BYTES-sized locals in mutually
exclusive branches of the same function -- dst in the sec->iter
two-phase path (which always returns before falling through) and
scratch in the direct sec->fill path. They are never simultaneously
live, but at -Og (CONFIG_COMPILER_OPTIMIZATION_DEBUG=y) the compiler
does not coalesce same-sized locals in disjoint lexical scopes of one
function -- it reserved stack for both, making this the largest frame
in the whole httpd call graph (measured 3344B via DWARF
DW_CFA_def_cfa_offset) and overflowing CONFIG_BB_HTTP_TASK_STACK_SIZE
on a real GET /api/diag/meminfo request (reproduced twice on hardware).

Replaced both locals with one render_scratch buffer declared once at
function scope and reused by whichever branch runs, so the sharing is
explicit rather than relying on scope-exit reuse the compiler doesn't
perform. Re-measured frame: 3344B -> 1808B, a 1536B drop (exactly
BB_DIAG_SECTION_SCRATCH_BYTES), confirming the double reservation is
gone. This dispatcher backs the single wildcard route for every
/api/diag/* section, so the fix benefits all of them, not just meminfo.

No wire-format change: same buffer size, same fill/iter/serialize call
order: all 5293 host tests pass unchanged, including the diag wire-
schema golden-content tests.

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