upgrade: Split deps and find older version better

[HaraldNordgren]

Group brew upgrade --dry-run dependencies into separate "Would install" and "Would upgrade" sections aligned with format_upgrade_summary, and read the installed version from the kegs so unlinked upgrades still show "old -> new".

Also stop the first dependency in the "Installing dependencies for" line from rendering bolder than the rest.

▶ brew upgrade --dry-run ffmpeg
...

==> Would install 11 dependencies for ffmpeg:
expat        2.8.2
meson        1.11.1
nasm         3.02
dav1d        1.5.3
libvmaf      3.2.0
libvpx       1.16.0
opus         1.6.1
sdl3         3.4.12
sdl2-compat  2.32.70
svt-av1      4.1.0
x265         4.2
==> Would upgrade 9 dependencies for ffmpeg:
ninja        1.10.2_1 -> 1.13.2
openssl@3    3.6.2    -> 3.6.3
readline     8.1.2    -> 8.3.3
sqlite       3.38.0   -> 3.53.3
xz           5.4.1    -> 5.8.3
lz4          1.9.4    -> 1.10.0
zstd         1.5.2    -> 1.5.7_1
python@3.14  3.7.1    -> 3.14.6
x264         r2854    -> r3222

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• AI was used to generate or assist with generating this PR.

Written by Claude Code and I iterated on in many times to get what I wanted.

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[MikeMcQuaid]

Looks good, small suggested style tweak and needs to use upgrade to avoid brew update confusion.

[HaraldNordgren]

@MikeMcQuaid Thanks, updated!

[MikeMcQuaid]

Thanks!