Commit f88a27b raised PERSIST_STORAGE_MAX_SPACE from 6 KiB to 1 MiB,
but settings_file_open pre-allocates the PFS file at ~120% of
max_used_space. This means every persist file now claims ~1.2 MiB of
flash regardless of actual usage, while most apps use only a few hundred
bytes.

Decouple the enforcement cap from the physical allocation by adding an
alloc_used_space field to SettingsFile that tracks the physical
allocation budget separately from max_used_space. A new
settings_file_open_growable() API sets alloc_used_space to a small
initial value. When a write needs more physical space but fits within
the enforcement cap, the file is automatically grown via rewrite with
doubling allocation (e.g. 4K -> 8K -> 16K -> ... -> 1M).

The persist service now uses settings_file_open_growable() with a 4 KiB
initial allocation (~5 KiB physical), close to the old 6 KiB limit.
Existing callers of settings_file_open() are unchanged (alloc_used_space
equals max_used_space, preserving the old pre-allocation behavior).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>