Add 'stacks' subcommand for folded/flame-graph output (#289)

[ChrisJr404]

Adds a stacks subcommand that prints the whole traversal as folded stacks, so a dua run can be turned into a disk-usage flame graph without leaving the tool. This implements the request in #289.

The folded (a.k.a. "collapsed") format is what flame-graph tools like inferno and Brendan Gregg's flamegraph.pl read on stdin: one line per entry, its path with ; between components, a space, and its size in bytes. That makes the usual pipe work:

dua stacks | inferno-flamegraph > disk-usage.svg

A couple of design notes:

• Rather than only emitting files (as Gregg's files.pl does), every entry gets a line carrying its own size: a directory contributes just the size of its directory entry, because the bytes it contains already show up on the lines of the things inside it. That avoids double-counting when the flame-graph tool sums a stack against its parents, and it means the folded lines sum to exactly the same total the other commands report — I added a test that checks this against a fresh traversal. Zero-sized entries are left out since they add nothing to a graph.
• It reuses the existing traversal, so all the traversal options carry over (--apparent-size, --ignore-from, --count-hard-links, threads, and so on). Hard links are counted once by default, as elsewhere.
• Entry names have the ; frame separator and any control characters replaced with _ so a single odd filename can't corrupt the line-based format.

I went with a subcommand rather than a flag on aggregate because the folded output needs the full in-memory tree, which aggregate doesn't build — but I'm happy to reshape the surface (name, flag vs. subcommand) if you'd prefer something else.

Example output:

$ dua stacks -A some_dir
a 80
a;big 5000
a;b 60
a;b;small 100
top 3

Testing: added four unit tests in src/stacks.rs covering the nested-file layout, the sum-equals-total invariant, a single-file input, and ;-in-name sanitization. cargo test is green (77 tests), cargo +nightly fmt --check is clean, and cargo +nightly clippy produces no new warnings from this change. Also smoke-tested by hand that dua stacks -A and dua aggregate -A --format bytes report the same total, and that the output pipes cleanly.

[Byron]

Can you please upload the SVG to this PR? I want to take a look. Please beware of PII. Thanks so much.