feat: forward-by-default, auto recursive mode, Linux install fixes (#27)

* feat: auto recursive mode, fix Linux install

Auto mode (new default): probes a root server on startup; uses
recursive resolution if outbound DNS works, falls back to Quad9 DoH
if blocked. Dashboard shows mode indicator (green/yellow).

Linux install fixes:
- Add DNSStubListener=no to resolved drop-in (frees port 53)
- Configure DNS before starting service (correct ordering)
- Skip 127.0.0.53 in upstream detection
- `numa install` now does everything (service + DNS + CA)
- `numa uninstall` mirrors install (stop service + restore DNS)
- Extract is_loopback_or_stub() for consistent filtering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: enable DNSSEC validation by default

With recursive as the default mode, DNSSEC validation completes the
trustless resolution chain. Strict mode remains off by default.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: forward search domains to VPC resolver on Linux

Parse search/domain lines from resolv.conf and create conditional
forwarding rules to the original nameserver or AWS VPC resolver
(169.254.169.253). Fixes internal hostname resolution on cloud VMs
where recursive mode can't resolve private DNS zones.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: single-pass resolv.conf parsing, eliminate redundancies

Parse resolv.conf once for both upstream and search domains instead
of 2-3 reads. Extract CLOUD_VPC_RESOLVER constant. Use &'static str
for mode in StatsResponse. Remove dead read_upstream_from_file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: macOS install health check, harden recursive probe

Verify numa is listening (API port) before redirecting system DNS on
macOS — if the service fails to start (e.g. port 53 in use), unload
the service and abort instead of breaking DNS. Probe up to 3 root
hints before declaring recursive mode unavailable. Validate IPs from
resolvectl to avoid IPv6 fragment extraction. Extract DEFAULT_API_PORT
constant.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: widen make_rule cfg gate to include Linux

make_rule was gated to macOS-only but discover_linux() calls it for
search domain forwarding rules. CI failed on Linux with E0425.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: forward mode as default, recursive opt-in

Forward mode (transparent proxy to system DNS) is now the default.
Recursive and auto modes are explicit opt-in via config. This avoids
bypassing corporate DNS policies, captive portals, VPC private zones,
and parental controls on first install.

- Move #[default] from Auto to Forward on UpstreamMode
- DNSSEC defaults to off (no-op in forward mode)
- 3-way match in main: Forward/Recursive/Auto with clean separation
- Post-install message suggests mode = "recursive" for sovereignty
- Update README, site, and launch drafts messaging

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -8,7 +8,7 @@
 
 A portable DNS resolver in a single binary. Block ads on any network, name your local services (`frontend.numa`), and override any hostname with auto-revert — all from your laptop, no cloud account or Raspberry Pi required.
 
-Built from scratch in Rust. Zero DNS libraries. RFC 1035 wire protocol parsed by hand. Recursive resolution from root nameservers with full DNSSEC chain-of-trust validation. One ~8MB binary, everything embedded.
+Built from scratch in Rust. Zero DNS libraries. RFC 1035 wire protocol parsed by hand. Caching, ad blocking, and local service domains out of the box. Optional recursive resolution from root nameservers with full DNSSEC chain-of-trust validation. One ~8MB binary, everything embedded.
 
 ![Numa dashboard](assets/hero-demo.gif)
 
@@ -24,7 +24,7 @@ sudo numa                              # port 53 requires root
 
 Open the dashboard: **http://numa.numa** (or `http://localhost:5380`)
 
-Set as system DNS: `sudo numa install && sudo numa service start`
+Set as system DNS: `sudo numa install`
 
 ## Local Services
 
@@ -43,7 +43,7 @@ Add path-based routing (`app.numa/api → :5001`), share services across machine
 
 385K+ domains blocked via [Hagezi Pro](https://github.com/hagezi/dns-blocklists). Works on any network — coffee shops, hotels, airports. Travels with your laptop.
 
-Two resolution modes: **forward** (relay to Quad9/Cloudflare via encrypted DoH) or **recursive** (resolve from root nameservers — no upstream dependency, no single entity sees your full query pattern). DNSSEC validates the full chain of trust: RRSIG signatures, DNSKEY verification, DS delegation, NSEC/NSEC3 denial proofs. [Read how it works →](https://numa.rs/blog/posts/dnssec-from-scratch.html)
+By default, Numa forwards to your existing system DNS — everything works as before, just with caching and ad blocking on top. For full privacy, set `mode = "recursive"` — Numa resolves directly from root nameservers. No upstream dependency, no single entity sees your full query pattern. DNSSEC validates the full chain of trust: RRSIG signatures, DNSKEY verification, DS delegation, NSEC/NSEC3 denial proofs. [Read how it works →](https://numa.rs/blog/posts/dnssec-from-scratch.html)
 
 ## LAN Discovery