docs: add ARCHITECTURE, ROADMAP, CITATION, FUNDING, editorconfig (#62)

Five governance + onboarding files that collectively close the
"no high-level documentation outside of README" gap.

- ARCHITECTURE.md: workspace layout, Indicator trait contract,
  per-indicator file conventions, numerical-stability notes,
  cross-crate flow diagram, navigation cheatsheet, deliberate
  non-goals, performance characteristics, stability commitments.
- ROADMAP.md: north star, 0.3 / 0.4 / 0.5 release windows,
  indicator-wishlist intake rules, explicit non-goals, versioning policy.
- CITATION.cff: GitHub-renders as "Cite this repository" button;
  enables academic adoption.
- .github/FUNDING.yml: surfaces Sponsor button on the repo page.
- .editorconfig: normalises indent/EOL across IDEs (Rust/Python 4 spaces,
  JS/TS/JSON/YAML/MD 2 spaces, Makefile tabs).

Touches no Rust source, no CI workflows, no behaviour. Additive only.
URLs in ROADMAP/CITATION already reference the post-transfer
`wickra-lib/wickra` org so the files stay valid once the org migration
in PR #59 lands; merge this PR only after #59 to keep main consistent.

.editorconfig

@@ -0,0 +1,34 @@
+# EditorConfig: https://editorconfig.org
+# Keeps indentation and line-endings consistent across IDEs.
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+
+# Rust + Python + most config files use 4-space indents.
+[*.{rs,py,toml}]
+indent_size = 4
+
+# JS / TS / JSON / YAML / Markdown use 2-space indents per ecosystem conventions.
+[*.{js,ts,jsx,tsx,json,yml,yaml,md}]
+indent_size = 2
+
+# Markdown allows trailing whitespace as a hard line break — keep it intact.
+[*.md]
+trim_trailing_whitespace = false
+
+# Makefiles must use tabs.
+[Makefile]
+indent_style = tab
+
+# Generated files are not authored by humans; leave them alone.
+[bindings/node/index.{js,d.ts}]
+indent_style = unset
+indent_size = unset
+trim_trailing_whitespace = unset
+insert_final_newline = unset